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CHAPTER 8

Subnetting: Networking AV Devices Together

In this chapter, you will learn about

• Different types of network segments, including subnets and VLANs

• How to subnet a classful network using subnet masks

• Why organizations would want to subnet in the first place

• Subnetting for IPv4 and IPv6

• How to figure out the subnets and hosts a network can support

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Networks are physically segmented into what are called broadcast and collision domains, but further subdivision is often necessary, especially when an organization wants to run an AV system on its existing network infrastructure. As networks get bigger—and the systems attached to them become hungrier for bandwidth—this logical subdivision becomes more important.

It so happens that real-time media, particularly video, has a voracious appetite for bandwidth. Not to mention, as AV professionals understand well, IT departments can be reluctant to introduce AV systems into their networks for fear of hindering overall performance. Subnetting represents a built-in subdivision mechanism for IP networks.

IP networking uses two addresses to identify a system: the IP address and the subnet mask. By altering the subnet mask, a single physical network can be transformed into many logical ones.

AV professionals working in a network environment need to be well versed in subnetting—it’s likely to be a critical step in getting their systems up and running.

Network Segments

A network segment is any section of a network that is physically separated from the rest of the network by a device such as a switch, router, or hub. It may contain one or more hosts. There are three main types of network segments: collision domains, broadcast domains, and—most important to networked AV systems—subnets.

Segmenting a network improves efficiency and security. After all, it makes sense for devices that communicate mostly with each other to be placed together. That way, communication among them doesn’t take up bandwidth that’s needed by other devices, nor does their data collide with the data of others. (See where we’re going with this? If AV devices get their own segment, they don’t get in the way of IT systems.)

Collision Domains

Collision domains are groups of devices on a LAN whose packets may collide if they’re sent at the same time. Going back to the OSI model, collision domains extend across Layer 1 devices and are separated by Layer 2 devices. The size of a network’s collision domains (and a LAN can have several) depends on the devices used.

For example, all network segments connected to a hub or wireless access point (WAP) belong to a single collision domain. These are Physical Layer devices that send the data they receive to all ports, including the one they received it from. If a hub or WAP receives data from more than one segment at a time, collisions may occur, interrupting data flow.

Every port on a switch is a separate collision domain. Therefore, a 24-port switch can have 24 separate collision domains. A switch is a Layer 2 device and forwards data only to the specified MAC address. Even if the MAC address isn’t known, a switch sends data to every port except the one through which it received the data. Devices on different network segments can communicate simultaneously through the switch.

Broadcast Domains

A broadcast domain is a set of devices that can send Data Link Layer frames to each other directly, without passing through a Network Layer device. Broadcast traffic sent by one device in the broadcast domain is received by every other device in the domain.

Broadcast domains can extend across Layer 2 devices; they’re separated by Layer 3 devices. All ports on a switch belong to the same broadcast domain (unless they’re separated by a virtual LAN). Each network interface on a router is a separate broadcast domain.

Subnets

A subnet is a Layer 3 concept. Subnets allow a single physical network to be split into many smaller logical networks. Using the subnet mask, as described in Chapter 7, you can divide collision domains, broadcast domains, or an entire LAN into smaller pieces to help increase efficiency or control bandwidth use.

A subnet is a logical group of hosts within a LAN. A LAN may consist of a single subnet or it may be divided into several subnets. Each subnet typically requires its own router. Additional subnets may be created by modifying the subnet mask on the network devices and hosts. Subnetting is an important part of configuring networked AV systems because it helps ensure that AV devices get the bandwidth they need for optimal performance while also satisfying the demands of IT departments that AV systems not interfere with other network functions.

Virtual LANs

Sometimes clients want certain users to have access to their networked AV traffic, but not their other resources. For example, a law firm may want to give its clients access to its videoconferencing system, but not its file servers. Therefore, you need to be able to subdivide subnets. Virtual local area networks (VLANs) provide a means for this subdivision and allow the AV system to leverage an enterprise’s existing network infrastructure, while still preserving its bandwidth and network security.

Because VLANs are not related to the network’s physical topology, devices on the same VLAN can be located anywhere on the LAN. Also, while a device can belong to only one subnet, it may belong to multiple VLANs. On many devices, a VLAN can be configured with an access control list, which specifies the devices allowed to access the VLAN or the devices the VLAN is allowed to access. The former protects data on the VLAN; the latter keeps devices on the VLAN from being used as a launching point for attacks.

Implementing a VLAN can be labor intensive because the VLAN broadcast domain must be segmented on every switch in the network. The network administrator must specify the MAC address, port, or IP address of every device on the VLAN, on every switch in the network.

A VLAN is a bit like a “friends and family” long-distance calling plan. Devices that need to “talk” frequently get efficient, unfettered access to one another. Their access to devices outside their VLANs, however, is slower and may be limited or even blocked. Because the communication among devices on a VLAN is typically switched rather than routed, it’s efficient.

Digital signage, for example, is well suited to a VLAN. If you have a campus-wide deployment of signs that all draw their content from a centrally located digital signage player, those devices probably belong on a VLAN. You’re unlikely to access other systems, such as file or mail servers, from your signage players or displays. Still, if you do want to send information from a database or directory to your signs, they can be accessed via the router.

Moreover, some AV-specific data link protocols don’t play well with others. CobraNet, for instance, is not designed to share network segments with other types of data. Placing all CobraNet devices on a single VLAN, however, isolates them on a broadcast domain. This leverages the existing physical infrastructure while putting the CobraNet traffic in virtual solitary confinement.

Requesting a VLAN

If you determine that an AV system should be segregated on a VLAN—for its own good or for the good of the network—you’ll need to provide the network manager with the following information:

• What VLAN you want to create and why (e.g., “I want to create an IPTV VLAN so that the network isn’t flooded with streaming video traffic.”)

• Which devices should be included in the VLAN

• Whether any routing between the VLAN and other network locations should be required or permissible

All this information should be documented in your networked device inventory (see Chapter 9).

Subnetting

Subnetting is the process of dividing a classful network into several smaller networks, called subnets. Subnets are created when a netmask is extended. In other words, instead of stopping at the end of an octet, the 1s in the netmask spill over to the next octet. For each bit the netmask is extended, the network is divided in half. If the netmask is extended 1 bit, you end up with two subnets. If it’s extended 2 bits, each subnet is divided again and you end up with four subnets. See Figure 8-1.

Note that as you divide the network, the subnets end up slightly less than half the size of the original network. That is, you can have slightly less than half the number of devices with global IP addresses. The reason you have less than half the addresses, and not exactly half, is that two of the addresses for each network can’t be public addresses. In order for any IPv4 network address to be valid, it needs three components: a subnet mask, a network address, and a broadcast address.

The subnet mask is a separate address and doesn’t affect the overall number of available addresses. However, the network address and the broadcast address are both within the network’s address range. The first address in the range—all 0s in the host bits—is the network address. The last address in the range—all 1s in the host bits—is the broadcast address. Therefore, each subnet has half the possible addresses of the undivided network, minus two.

The Subnet Mask

Subnet masks were introduced with the advent of CIDR, under which the number of network bits is determined not by class but by a slash and number at the end of the IP address. Today subnetting is so common that the networking community uses the terms “subnet mask” and “netmask” interchangeably.

In IPv4, the subnet mask is a 32-bit number expressed in dot-decimal notation, just like an IP address. In a binary representation of the subnet mask, bits equal to 1 indicate that the corresponding bits in the IP address identify the network. Bits equal to 0 indicate that the corresponding bits in the IP address identify the host. IP addresses with the same network identifier bits as those identified by the subnet mask are on the same subnet.

In order to extend the subnet mask, and therefore create a subnet, bits are “borrowed” from the host identifier and added to the network identifier. Every time the network is subnetted, you have fewer bits with which to identify devices on each subnet. See Figure 8-2. The more IPv4 subnets you have, the fewer devices you can put on each.

In IPv6, the fourth hexadecimal word in the IP address is set aside for defining subnets. The subnet may therefore be expressed as either a full sixteen-character hexadecimal address or a single four-character hexadecimal word. These dedicated bits allow IPv6 networks to be subnetted without having to “borrow” any host identifier bits. See Figure 8-3.

Using all 16 bits set aside for subnetting, you can create more than 65,000 subnets without reducing the number of devices that can be put on each network. An IPv6 network with a single subnet can contain 2⁶⁴ devices. If that network is divided into 100 different subnets, each subnet can still contain 2⁶⁴ devices. It is technically possible to use device identifier bits to extend the IPv6 subnet mask, but doing so might prevent the use of IPv6 autoconfiguration tools. And really, who needs 130,000 subnets?

Subnetting in Practice

Internet service providers (ISPs) often subnet classful IPv4 addresses before assigning them to an organization. This allows ISPs to make better use of the limited number of available classful addresses. For example, say an ISP owns the class B network 128.162.0.0. That ISP can assign that address to a single organization as 128.162.0.0/16. In this case, the subnet mask would be the class B netmask—255.255.0.0—because the network hasn’t been divided.

The organization can have up to 65,534 public addresses on that network. However, the ISP could also choose to divide the network and assign portions of it to different organizations. All the ISP has to do is extend the subnet mask to use more of the IP address’s bits as network identifiers. The ISP could divide the network into two subnets by extending the netmask from /16 to /17. It could then assign each subnet to a different organization, as shown in Table 8-1.

The ISP could further divide the network into four subnets by extending the netmask from /16 to /18. The ISP could then assign each subnet to a different organization, as shown in Table 8-2.

Often an ISP will extend the netmask of a class B address to /24. That means the subnet extends across the entire third octet. As a result, the class B network is divided into 256 subnets, each with up to 254 public addresses. This way, the ISP can provide connections to 256 organizations instead of just one.

Subnetting within an Organization

ISPs subnet to conserve IPv4 addresses, but private organizations subnet, too, though not for the same reasons. Subnetting an organization’s network actually reduces the number of unique addresses it can use per subnet, so why would it divide its network? To increase network efficiency.

Subnetting doesn’t actually boost the amount of available bandwidth on a network, but it does allow devices to make better use of the available capacity. A subnet has fewer addresses than an undivided network. This allows address resolution protocol to resolve addresses faster, speeding up communication within the subnet. Fewer devices also mean less broadcast traffic—devices on a subnet only receive broadcast messages from other devices on the same subnet, not from the whole undivided network.

An organization may also subnet for security purposes. For example, say you have a full class C address, with 254 possible global addresses, but only your router and your web server actually need direct Internet connections. You have 252 unused addresses lying around that someone could use to register their own device to your network. If you extend your subnet mask to /30, however, there are suddenly only two possible public addresses. Assign one to your web server and the other to your router, and there’s no room for anyone else.

Calculating IPv4 Subnets: CIDR Notation

When you look at an IPv4 subnet mask, how can you tell how many subnets the network has been divided into? If the subnet mask is indicated in CIDR notation, it’s easy to figure out.

Formula: Calculating the Number of Subnets in a Network

The formula for the number of subnets in a network is

Subnets = 2^{x – y}

where

• Subnets is the number of subnets the network has been divided into

• x is the number of network identifier bits in the subnet mask

• y is the number of network identifier bits in the unextended netmask

CIDR notation tells you how many bits the subnet mask “masks.” That is, if you write the subnet mask out in binary, how many of its most significant bits will be 1s? Determining the number of subnets a CIDR subnet mask represents requires two steps:

• Step 1: Determine the number of network identifier bits in the original, undivided network.

• Step 2: Use the formula for calculating the number of subnets in an IPv4 network to solve for the number of subnets.

Let’s look at an example: Cherry Blossom Event Services has been assigned a global IP address of 218.35.172.0/26 and decides to subnet its network by extending the netmask to /28. How many subnets does Cherry Blossom Event Services now have?

• Step 1: Determine the number of network identifier bits in the undivided network. Cherry Blossom Event Services’ original network address was 218.35.172.0/26. CIDR notation indicates that this network has 26 network identifier bits.

• Step 2: Use the formula to calculate the number of subnets.

• Subnets = 2^{x – y}

• Subnets = 2^{28 – 26}

• Subnets = 2²

• Subnets = 4

By extending its netmask to /28, Cherry Blossom Event Services divided its network into four subnets.

Let’s try a harder problem: Southeastern Big Data Systems has been assigned a global IP address of 202.168.64.0/25. It would like to divide that network into eight subnets. What should its new subnet mask be?

• Step 1: Determine the number of network identifier bits in the undivided network. Southeastern Big Data Systems’ original network address was 202.168.64.0/25. CIDR notation indicates that this network has 25 network identifier bits.

• Step 2: Use the formula to calculate the number of network bits in the subnet mask.

• Subnets = 2^{x – y}

• 8 = 2^{x – 25}

• log₂(8) = log₂(2^{x – 25})

• 3 = x – 25

• 3 + 25 = (x – 25) + 25

• 28 = x

In order to divide its network into eight subnets, Southeastern Big Data Systems should extend its subnet mask to /28.

Calculating IPv4 Subnets: Dot-Decimal Notation

If the subnet mask is written out in dot-decimal notation, rather than CIDR notation, you have to do a little more work to determine the number of subnets in a network. The formula remains the same, but you have to convert the subnet masks to binary before you can determine the number of network identifier bits.

Determining the number of subnets that a dot-decimal subnet mask represents takes five steps:

• Step 1: Convert the original netmask to binary.

• Step 2: Determine the number of network identifier bits in the original, undivided network.

• Step 3: Convert the subnet mask to binary.

• Step 4: Determine the number of network identifier bits in the subnet mask.

• Step 5: Use the formula for calculating the number of subnets in an IPv4 network to solve for the number of subnets.

Let’s try it out: Briar Rose Sports Bar has been assigned a global IP address of 220.32.128.0. Its subnet mask is 225.225.225.192. The company decides to subnet its network by extending its netmask to 225.225.225.248. How many subnets does Briar Rose Sports Bar have?

• Step 1: Convert the original netmask to binary (get out those calculators).

• 255.255.255.192 = 11111111 11111111 11111111 11000000

• Step 2: Determine the number of network identifier bits in the undivided network. Count the number of 1s in the most significant bits (msbs, or leftmost bits) of the binary subnet mask.

• 11111111 11111111 11111111 11000000 (the undivided network has 26 network bits)

• Step 3: Convert the subnet mask to binary.

• 255.255.255.248 = 11111111 11111111 11111111 11111000

• Step 4: Determine the number of network identifier bits in the undivided network. Count the number of 1s in the most significant bits of the binary subnet mask.

• 11111111 11111111 11111111 11111000 (the subnet mask has 29 network bits)

• Step 5: Use the formula to calculate the number of subnets.

• Subnets = 2^{x – y}

• Subnets = 2^{29 – 26}

• Subnets = 2³

• Subnets = 8

Briar Rose Sports Bar has divided its network into eight subnets.

Calculating Hosts on IPv4 Subnets

As you divide an IPv4 network, the pool of potential addresses shrinks. Given a subnet mask, you can calculate the number of unique host addresses each subnet has.

Formula: Calculating the Number of Hosts in an IPv4 Subnet

The formula for the number of host addresses in an IPv4 subnet is

Hosts = 2^x – 2

where

• Hosts is the number of unique host addresses available on each subnet

• x is the number of host identifier bits in the subnet mask

The 2^x tells you how many different possible combinations of 1s and 0s there are in the host bits. Remember, though: you can’t use all 1s or all 0s as a host address. That’s why you subtract two from the total.

How do you know how many host bits a subnet mask has? In CIDR notation, it’s easy. Every IPv4 address has 32 bits total. Just subtract the CIDR prefix from the number of network bits and you get the number of host bits. If the subnet mask is written in dot-decimal, just convert the mask to binary and count the number of 0s.

How many hosts can be on each subnet of a network with a /13 CIDR prefix?

• Step 1: Determine the number of host identifier bits.

• 32 – 13 = 19

• Step 2: Use the formula to determine the number of host addresses.

• Hosts = 2^x – 2

• Hosts = 2¹⁹ – 2

• Hosts = 524,288 – 2

• Hosts = 524,286

There can be 524,286 hosts on each subnet.

Time for another example: Perry Consulting has a class C network. It wants to subnet its network so it can put 14 devices on each subnet. What should its network CIDR prefix be?

• Step 1: Use the formula to determine the number of host bits.

• Hosts = 2^x – 2

• 14 = 2^x – 2

• 14 + 2 = (2^x – 2) + 2

• 16 = 2^x

• log₂(16) = log₂(2x)

• 4 = x

There must be four host bits in each address.

• Step 2: Determine the number of network identifier bits.

• 32 - 4 = 28

The CIDR prefix is /28.

Determining Subnets for IPv4 Addresses

Once you subnet an IPv4 network, how do you know which addresses belong to which subnets? Remember, if two devices share the same network bits, they’re on the same network. A subnet mask identifies extra network bits. The easiest way to determine if two IPv4 addresses are on the same subnet is to convert both addresses to binary. If they have the same network bits, they are on the same subnet.

Are the IP addresses 192.220.38.15/25 and 192.220.38. 240/25 on the same subnet?

• Step 1: Convert the IP addresses to binary.

• 192.220.38.15 = 11000000 11011100 00100110 00001111

• 192.220.38.240 = 11000000 11011100 00100110 11110000

• Step 2: Compare the addresses’ network bits. The 25 most significant bits are network bits.

• 11000000 11011100 00100110 00001111

• 111000000 11011100 00100110 11110000

The last network bit in these two addresses is different. These addresses are not on the same subnet.

 

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NOTE You can also calculate the address range of each subnet. If two addresses are within the same address range, they are on the same subnet. The only way to reliably calculate subnet ranges for classless addressing, though, is to write out each network address in binary and then convert it to decimal. If you’re dealing with more than a handful of subnets, that’s an unnecessarily time-consuming process. Instead, you can use one of many free subnet address range calculators, available both online and as mobile apps. Find your own by searching for “IP subnet calculator.”

IPv6 Subnetting

You can subnet an IPv6 network just like an IPv4 network. However, the reasons for doing so are different. There are so many IPv6 addresses that you would never subnet a network to conserve addresses.

When subnetting an IPv4 network, networking professionals ask themselves, “How many hosts do I need to put on each subnet?” With IPv6, you can have a virtually unlimited number of hosts on any given network, so the question instead is, “How many subnets do I want?”

Other than address conservation, the reasons to subnet devices on an IPv6 network are pretty much the same as the reasons to do so on an IPv4 network: increasing network efficiency, decreasing broadcast traffic, and so on.

IPv6 Subnet Masks

According to the recommendations of the IETF, an IPv6 subnet mask typically covers only the fourth word of an IP address. That leaves the first three words free to identify the network and the last four words free to identify the hosts. IPv6 subnet masks are usually identified by CIDR notation, but you can also write them out as IPv6 addresses. Table 8-3 shows all possible IPv6 subnet masks, with the most commonly used masks in bold.

Technically, you could use any subnet mask between /48 and /64 to subnet an IPv6 address. Remember, however, that each hexadecimal character represents 4 bits, known as a nibble. In practice, most networking professionals subnet along the nibble, extending the mask 4 bits each time they divide the network. This is why /48, /52, /56, /60, and /64 are the most common IPv6 CIDR prefixes. Subnetting this way wastes a lot of network addresses, but there are so many possible IPv6 network addresses that no one really cares.

Calculating Subnets for IPv6 Networks

The process for calculating the number of subnets for IPv6 is the same as for IPv4, so let’s give it a shot.

An ISP assigns FutureTech Software the IPv6 network address 7a12:9a:114b::/48. FutureTech extends its subnet mask to /56. How many subnets does FutureTech have?

• Step 1: Determine the number of network identifier bits in the undivided network. FutureTech Software’s original network address was 7a12:9a:114b::/48. CIDR notation indicates that this network has 48 network identifier bits.

• Step 2: Use the formula to calculate the number of subnets.

• Subnets = 2^{x – y}

• Subnets = 2^{56 – 48}

• Subnets = 2⁸

• Subnets = 256

By extending its netmask to /56, FutureTech Software divided its network into 256 subnets.

Now let’s work it the other way around. Pinnacle Technologies has been assigned an IPv6 address of e0c7:ff16:119c:7b00::/56. The company would like to divide its network into 16 subnets. What should its new subnet mask be?

• Step 1: Determine the number of network identifier bits in the undivided network. Pinnacle Technologies’ network address is e0c7:ff16:119c:7b00::/56. CIDR notation indicates that this network has 56 network identifier bits.

• Step 2: Use the formula to calculate the number of network bits in the subnet mask.

• Subnets = 2^{x – y}

• 16 = 2^{x – 56}

• log₂(16) = log₂(2^{x – 56})

• 4 = x – 56

• 4 + 56 = (x – 56) + 56

• 60 = x

In order to divide its network into 16 subnets, Pinnacle Technologies should extend its subnet mask to /60.

Calculating Hosts on IPv6 Subnets

You never have to worry about how many potential hosts are on an IPv6 subnet, because the number of host bits never changes. You always use the last four words to identify the hosts, meaning you always have 64 host bits. As a result, no matter how many times you divide an IPv6 network, each subnet has more than 18 quintillion addresses. How can you tell, though, if IPv6 addresses are on the same subnet? This is actually much easier than under IPv4.

If the first four words of any IPv6 addresses are the same, then those addresses are on the same subnet. If they’re different, then they’re not on the same subnet. So, are the following addresses on the same subnet?

• 7a12:9a:114b:78c0::32/60

• 7a12:9a:114b:78d0:ab72:9c:1c81:1/60

Just compare the first four words in the two addresses. As you can see, 78c0 and 78d0 (the fourth words in the respective IPv6 addresses) are not the same, therefore these two devices are not on the same subnet.

Chapter Review

As alluded to in Chapter 7, if a networked AV system comprises devices that need to communicate with one another using Ethernet, they should be on the same subnet. And if they’re to do so efficiently, at a high level of performance and without interfering with IT systems, it might make sense to put them on their own subnet. Understanding subnetting and subnet masks is a fundamental skill for ensuring that networked AV systems get the IP addresses they need.

Now that you’ve completed this chapter, you should be able to

• Identify different types of network segments, including subnets

• Understand the importance and benefit of subnetting to networked AV systems

• Identify the number of subnets and hosts for a IPv4 network

• Identify the number of subnets and hosts for a IPv6 network

Review Questions

1. Implementing a virtual local-area network can be labor intensive because _______.

A. its broadcast domain has to be segmented on every switch in the network

B. you may be limited to whatever addressing scheme the client already use

C. it increases bandwidth overhead by adding an encryption and tunneling wrapper

D. you will have to manually set IP addresses for each device

2. Subnetting is the process of ______.

A. logically dividing a network into several smaller networks

B. decreasing the size of a network by changing the network class

C. identifying the network bits in an IP address

D. configuring virtual private LANs on a network switch

3. If you subnet an IPv4 network by extending the subnet mask by 2 bits, you end up with _______.

A. two subnets, each about one-half the size of the original network

B. four subnets, each about one-half the size of the original network

C. four subnets, each about one-quarter the size of the original network

D. two subnets, each the same size as the original network

4. A private organization might wish to subnet its own network in order to _______. Select all that apply.

A. increase total network bandwidth

B. decrease the amount of broadcast traffic devices receive

C. increase the size of its host address pool

D. improve network security

E. speed up ARP resolutions

5. True or false? The IP addresses 192.220.38.15/25 and 192.220.38.240/25 are on the same subnet.

A. True

B. False

6. True or false? Given a subnet mask of 255.255.240.0, 137.19.12.1 and 137.19.15.254 are on the same subnet.

A. True

B. False

7. An organization might choose to subnet an IPv6 network in order to _______. Select all that apply.

A. decrease the amount of broadcast traffic each device receives

B. conserve the overall number of available IPv6 addresses

C. increase the efficiency of ARP resolutions

D. increase network bandwidth

8. True or false? You can calculate the number of subnets in an IPv6 network using the same formula as for an IPv4 network.

A. True

B. False

9. If you subnet an IPv6 network by extending the subnet mask from /48 to /52, you end up with ____.

A. four subnets, each with the same number of host addresses as the original network

B. 16 subnets, each with 1/16 the number of host addresses as the original network

C. 16 subnets, each with the same number of host addresses as the original network

D. four subnets, each with one-quarter the number of host addresses as the original network

10. True or false? If the first three words of two IPv6 addresses are the same, then those two addresses are on the same subnet.

A. True

B. False

Answers

1. A. Implementing a virtual local-area network can be labor intensive because its broadcast domain has to be segmented on every switch in the network.

2. A. Subnetting is the process of logically dividing a network into several smaller networks.

3. C. If you subnet an IPv4 network by extending the subnet mask by 2 bits, you end up with four subnets, each about one-quarter the size of the original network.

4. B, D, E. A private organization might wish to subnet its own network in order to decrease the amount of broadcast traffic devices receive, improve network security, and speed up ARP resolutions. Subnetting doesn’t increase bandwidth.

5. B. False. Convert the addresses to binary and compare the 25 most significant bits (msbs), and you can see that they are not on the same subnet.

6. A. True. With a subnet mask of 255.255.240.0, devices with IP addresses of 137.19.12.1 and 137.19.15.254 are on the same subnet.

7. A, C. An organization might choose to subnet an IPv6 network in order to decrease the amount of broadcast traffic each device receives or increase the efficiency of ARP resolutions. There are so many possible IPv6 addresses that conserving them isn’t a factor.

8. A. True. You can calculate the number of subnets in an IPv6 network using the same formula as for an IPv4 network.

9. C. If you subnet an IPv6 network by extending the subnet mask from /48 to /52, you end up with 16 subnets, each with the same number of host addresses as the original network.

10. B. False. If the first four words of two IPv6 addresses are the same, then those two addresses are on the same subnet.