IP Addressing: Network ID and Host ID

Here are the cold hard facts: An IP address is a 32-bit binary address. This 32-bit address is subdivided into four 8-bit segments called octets. Since only a fraction of people work with 32-bit binary addresses or even 8-bit binary octets (math and computer types), the IP address is almost always expressed in what’s called the “dotted decimal” format. What’s that? Well, if you go to the “WhatIsMyIPAddress.com” home page to check out your IP address, you’ll see it in dotted-decimal format. In dotted-decimal format, each octet is given as an equivalent decimal number. The four decimal values (4×8 = 32 bits) are then separated with periods. These numbers in an IP address are derived from eight binary bits, which can represent any whole number from zero to 255. Eight binary bits can represent any whole number from zero to 255, so the segments of a dotted-decimal address are decimal numbers with a range from 0 — 255. Every IP address (even though it looks to be in four parts) is broken down into two segments…but those segments aren’t equal. Part of the IP address is used for “network ID, and the rest of the address is used for the “host ID.” The host ID would identify your network connection, for example. Why are the segments not equal? Because address allotted to the network ID varies, depending on the address. Most IP addresses fall into the following address classes: More bits lead to more combinations. As a math-minded person might guess, the Class A format provides a small number of possible network IDs and a huge number of possible host IDs for each network. A Class A network can support approximately 224 hosts, which works out to 16,777,216 hosts. A Class C network can support, on the other hand, can provide host IDs for only a small number of hosts (approximately 28 hosts, or 256), but many more combinations of network IDs are available in the Class C format. You might be wondering how a computer or router knows whether to interpret an IP address as a Class A, Class B or Class C address. The designers of TCP/IP wrote the address rules in way that the address class is obvious for the address itself. The first few bits of the binary address specify whether the address should be interpreted as a Class A, B or C address: Here’s how that works: The simple table below shows the address ranges for Class A, B and C networks. Note that some addresses ranges are listed as “excluded.” That’s because certain IP address in each class are reserved for special uses (and a different IP address class) are therefore are not assigned to the Address Classes.) Address Ranges: Class A, B and C networks: In theory, only 127 Class A networks can exist on the Internet. (It’s all somewhat heady, and complicated, math stuff, so don’t bust your brain trying to figure out why.) However, each of those networks can have about 17 million hosts . Only a few very large organizations are Class A networks. Class B networks are good-sized networks. They are in more numbers that Class A, but the networks themselves are not as large as Class A networks are. A Class B network can have about 65,000 hosts (computers) connected to it. Most large companies and universities would fall into the size of network. Not to be left out, Class C networks are plentiful—there are more than 2 million of them. They’re the most common and they’re a lot smaller, one Class C network can support no more than 254 hosts; most networks that connect to the Internet are Class C.