The technical committees, international standards organizations, and government agencies that manage the Internet have all agreed on a 32-bit numeric address format shown as four numbers between 0 and 255, separated by periods, like this:

When you read an IP address out loud, you pronounce each digit separately and each period as "dot." So you would read this sample address as "one-nine-two dot one-six-eight dot three dot two-zero-zero."

You can think of an IP address as similar to your telephone number. Every computer connected to your LAN and every device or network connected to the Internet has a different address.

The agency responsible for assigning numeric IP addresses on the Internet is the Internet Assigned Names Authority (IANA). Some formal contracts with the US government are involved, but the real reason IANA can provide this service to the worldwide Internet community is that everybody agrees to respect their assignments.

As the owner of a small LAN, you will never deal directly with IANA. Your Internet service provider controls a block of numeric addresses, and it will assign you one address (or more) when you set up your new connections.

As Chapter 3 explained, your LAN communicates with other networks through a router. As far as the networks connected to that router are concerned, the router is just one more network connection with an IP address. Therefore, as Figure 4-1 shows, a router has two different IP addresses: one for its connection to the LAN and the other for the WAN or the Internet. The router presents a single address to the Internet that represents all the computers and other devices on your LAN; it performs a function called network address translation (NAT) that converts your public address to the addresses of individual network devices. One of the benefits of this system is that you can use the same IP addresses within your LAN as your neighbor across the street (or a LAN on the other side of the world), and the addresses won't interfere with one another.

In order to make this system work properly, IANA has reserved several blocks of IP address numbers for LANs; when a router receives a packet with an address in one of these ranges, it does not relay the packet to the Internet. If you use these addresses for the devices in your LAN, you can be certain that your packets (and the commands, messages, and files that make up those packets) won't end up at the reading room of the National Library of Ecuador when you wanted to send them to your assistant across the corridor.

The reserved IP addresses are:

The computers and other nodes in your LAN can obtain their numeric IP addresses in one of two ways: The person who sets up the network connection can assign a permanent address, or a router or other network control device can automatically assign an address every time the device connects to the network. A permanent assignment is called a fixed or static IP address; an automatic assignment is a dynamic address.

Figure 4-1. A router presents separate IP addresses to each network.

The method for assigning dynamic IP addresses is called Dynamic Host Configuration Protocol (DHCP), so the device that makes the assignments is a DHCP server. In a LAN, the DHCP server uses numbers from the reserved range; on the Internet, the servers use numbers from a range provided to your ISP by IANA.

Both fixed and dynamic IP address assignments can work equally well, but all the devices on the network must use the same system; otherwise, more than one device might use the same number at the same time.

Some Internet service providers and corporate network managers assign static IP addresses to each user, whereas others use DHCP to generate addresses. Chapter 10 and Chapter 11 explain how to set up your own computer and LAN to use either method.

Computers have no trouble handling long strings of numbers, but people often do. Addresses in the form of words rather than numbers are generally easier to remember and use. That's why the Internet and just about every LAN use names for each computer connected to a network. In a LAN, each computer reads the name of every other device on the same network automatically; on the Internet, a computer called a Domain Name System server (DNS server) converts names to numeric addresses; when you type the name of a website into a browser, a DNS server finds the number that corresponds to that name and returns it to your browser, which connects to that numeric address.

You (or your network manager) will assign a name to each computer when you set up your network; your Internet service provider should set up a domain name for your connection to the Internet. Within a LAN, you can use simple descriptive names for each computer, such as "Sam" or "Kate."

On the other hand, the system for naming computers and networks connected to the Internet (rather than to your own LAN) follows some very specific rules called the Domain Name System (DNS). In the Domain Name System, every name starts with a top-level domain name at the extreme right that can be either a generic description (such as com, net, or edu) or a two-letter country code (such as uk for the United Kingdom or ca for Canada). As you move to the left, the next word (or group of letters and numbers) is a name (called a subdomain) that has been reserved by a specific owner—an individual, a business, a government agency, or some other formal or informal organization. Large organizations might have one or more additional subdomain names to the left of the first one. Each part of the name is divided from the next one by a period (read as dot).

For example, the University of Washington's domain name is washington.edu. Within the university, the Department of Genome Science's address is gs.washington.edu. And within that department, the addresses of the research group studying evolutionary genetics is evolution.gs.washington.edu.

At the extreme left of a domain name, you will sometimes see a subdomain that identifies the type of server. This address might be the familiar www or some other Internet service such as ftp (file transfer protocol).

Many addresses also include the type of Internet service (the protocol) that the web resource at that address uses as a leading part of the address, followed by a colon and two forward slashes (//), such as http://host.sample.com/. The http part stands for HyperText Transfer Protocol —the protocol that defines most websites. If you want to reach a different service at the same destination such as a file transfer server, a telnet host, or an Internet Relay Chat server, you might instead use ftp://host.sample.com/, telnet://host.sample.com/, or irc://host.sample.com/, respectively. When an address does not include the protocol type and the two forward slashes, your web browser will assume it's an http address. Some top-level domains that use country codes have other structures that differ from one country to another. Domain names that have a us (for United States) top-level domain sometimes use subdomains (also called second-level domains) that identify the state and city where the owner is located, such as example.sf.ca.us, which would be in San Francisco, California. In Canada and other countries, the domain name comes right before the country code (such as the Canadian Broadcasting Corporation's cbc.ca), whereas other countries use generic identifiers along with the geographic domain, such as bbc.co.uk for the British Broadcasting Corporation; the co stands for commercial and the uk for the United Kingdom.

Table 4-1 lists the most common generic top-level domains.

Some other top-level domains such as .asia, .coop, .museum, and .travel are restricted to certain categories of users. Still others, such as

are for addresses that don't use the Roman alphabet.

DNS name servers are an essential part of the Internet's internal plumbing, but most people don't know that they exist. If your computer can't find a DNS server, your email program, web browser, and other Internet programs won't work unless you use a numeric IP address to identify a destination.

DNS servers perform what seems like a simple task, but this task is more complicated than it first appears because millions of domain names are out there, and new ones are added all the time. Every DNS server in the world has to keep up with all the adds, moves, changes, and deletions. It accomplishes this through a system of root servers that are continuously updated. If a local DNS server doesn't recognize a name, it consults the root server that keeps up with that name's top-level domain.

When you set up your computer for access to the Internet, you must specify the DNS servers that the computer will use to convert domain names to numeric IP addresses. In most cases, your Internet service provider or network manager will give you the numeric address of one or more nearby DNS servers. If your primary DNS server is not accessible, your computer will look for an alternate server if you have provided an alternate address.

It's generally best to use the DNS server address supplied by your ISP because the server with this address is probably closer to your own computer than any other server, and the system works best when total demand for DNS service is spread among as many servers as possible. But if you can't obtain reliable DNS service from your local service provider, a public DNS is often a useful alternative. You can find addresses for several public DNS servers though a Google or other web search for Public DNS server.

Some public DNS services can also provide some added features that your ISP might not offer. For example, OpenDNS (http://www.opendns.com/) can provide another layer of filtering against spyware, identity theft, adult sites, and other possible problems. It will also allow you to set up two three-letter shortcuts to frequently used addresses and will automatically correct common keystroke errors (such as typing example.cmo instead of .com). There's some controversy about some of these features, because they could lend themselves to returning names that are links to advertisements rather than the sites the original user requested.