Archive for the ‘NAT (Network Address Translation)’ Category

IP Addresses – Explained

Thursday, November 12th, 2009

IP Addresses.

IP (Internet Protocol) addresses are numbers that identify Internet hosts. They provide universal addressing across all the networks of the Internet.

IP addresses are placed in the IP packet header and are used to route packets to their destinations. An IP address is a 32-bit value split into four 8-bit pieces (octets) that are separated by dots. An example of an IP address is Each of the 4 numbers within the IP address can be between 1 and 255.

IP addresses are prefix based. The initial prefixes of the IP address can be used for generalised routing decisions. For example, the first 16 bits of an address might identify a corporation, the next 4 bits may identify a branch of that corporation, the following 6 bits may identify a particular LAN in that corporate branch, and the entire 32-bit address might identify a specific host within that LAN.

To simplify packet routing, Internet addresses are divided into five classes: Class A, Class B, Class C, Class D, and Class E. Very large corporations and entities receive Class A addresses, mid-sized companies and universities usually have Class B addresses, and most smaller companies and ISPs have Class C addresses. Class D is a multicast address and Class E is reserved.

Class A addresses are given to large organisations such as major universities and very large corporations. Class A addresses begin with a number between 1 and 126 (127 is reserved) in the first octet, leaving the 3 other octets open to split into local addresses. Although there are only 126 Class A codes, there are more than 16 million individual IP addresses within each Class A. Class B addresses are claimed by mid-sized companies, universities, and other entities that need thousands of IP addresses. Class B IP addresses begin with numbers between 128 and 191 in the first octet and have numbers from 1 through 255 in the second octet, leaving the last 2 octets open to denote local addresses. There are 16,384 Class B addresses with 65,536 individual IP addresses each.

Class C addresses —the most common—are used by most companies and ISPs. A Class C address has a number from 192 through 223 in the first octet and a number from 1 through 255 in the second and third octets, leaving only the fourth octet free for local addresses. There are more than two million Class C addresses and each contains 255 IP addresses.

Subnetting enables a network administrator to further divide the host part of the address into two or more subnets to make them easier to manage. A filter called a subnet mask is used to determine the subnet to which an IP address belongs.

Because IP addresses are difficult to remember, many also have text equivalents such as These text-based addresses are called domain names. A database program called Domain Name Service (DNS) keeps track of the names and translates them into their numeric equivalents.

The Internet is expected to outgrow the number of available IP addresses eventually. A new system of IP addresses called IPv6 has been designed to extend the capacity of the Internet. To date, the uptake of IPv6 has been limited. Most people are still using IPv4 and NAT (Network Address Translation) which allows multiple devices to connect to the Internet using only one ‘real’ IP address.

First Octet Second Octet Third Octet Fourth Octet