IP networks and subnets

Before moving on, a short review of typical IP subnetting terms and typical applications should help clarify the terms used in this book and will act as a refresher for those already versed in IP addressing.

A /24 designator placed after a network IP address in diagrams or device configurations is a Classless Inter-Domain Routing (CIDR) designator that indicates the following:

  • The first 24 out of the 32 bits in the 4-byte IP address represents the network portion of any IP address on this network. This network is designated as 10.1.1.0 (the next /24 network would be 10.1.2.0, then 10.1.3.0, and so on).
  • The last 8 bits of the 32-bit address can be used to give workstations, hosts, and other devices an IP address, with the following exceptions:
    • The first host address on this network is reserved as a network designator to build routing tables: 10.1.1.0 (typically called the loopback address)
    • The last host address on this network is reserved as an IP broadcast address: 10.1.1.255

    The 8 bits binary is equal to 256 decimal, minus the preceding two exceptions. This leaves 254 usable IP addresses for devices, starting with 10.1.1.1, 10.1.1.2, and so on up to 10.1.1.254.

  • Another way of expressing subnet masks is in a dotted decimal format, 255.255.255.0, which again indicates that the first 24 bits of an IP address is the network and the remaining 8 bits are for hosts.
  • There are Class A, Class B, and Class C address ranges, as well as a subset of IP ranges reserved as private addresses to use within organizations.

    The following table shows the IP address ranges in the three major classes:

    Class of IP address

    Starting IP address

    Ending IP address

    A

    1.0.0.0

    126.255.255.255

    B

    128.0.0.0

    191.255.255.255

    C

    192.0.0.0

    223.255.255.255

    The following table shows the private IP address ranges:

    Class of private IP addresses

    Starting IP address

    Ending IP address

    A

    10.0.0.0

    10.255.255.255

    B

    172.16.0.0

    172.32.255.255

    C

    192.168.0.0

    192.168.255.255

  • Subnet masks can be configured to allow more or fewer hosts per subnet with a corresponding tradeoff in having fewer or greater network addresses with which to build multiple networks within larger organizations.

A deeper review of IP addressing and subnetting is beyond the scope of this book. If you're not familiar with these concepts, some additional study would be advisable as a solid understanding of IP subnetting is essential for most analysis activities.

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