An IP address is an address used to uniquely identify a device on an IP network. The address is made up of 32
binary bits which can be divisible into a network portion and host portion with the help of a subnet mask. The
32 binary bits are broken into four octets (1 octet = 8 bits). Each octet is converted to decimal and separated
by a period (dot). For this reason, an IP address is said to be expressed in dotted decimal format (for example,
172.16.81.100). The value in each octet ranges from 0 to 255 decimal, or 00000000 − 11111111 binary.
Here is how binary octets convert to decimal: The right most bit, or least significant bit, of an octet holds a
value of 20. The bit just to the left of that holds a value of 21. This continues until the left−most bit, or most
significant bit, which holds a value of 27. So if all binary bits are a one, the decimal equivalent would be 255
as shown here:
1 1 1 1 1 1 1 1
128 64 32 16 8 4 2 1 (128+64+32+16+8+4+2+1=255)
Here is a sample octet conversion when not all of the bits are set to 1.
0 1 0 0 0 0 0 1
0 64 0 0 0 0 0 1 (0+64+0+0+0+0+0+1=65)
And this is sample shows an IP address represented in both binary and decimal.
10. 1. 23. 19 (decimal)
00001010.00000001.00010111.00010011 (binary)
These octets are broken down to provide an addressing scheme that can accommodate large and small
networks. There are five different classes of networks, A to E. This document focuses on addressing classes A
to C, since classes D and E are reserved and discussion of them is beyond the scope of this document.
Note: Also note that the terms "Class A, Class B" and so on are used in this document to help facilitate the
understanding of IP addressing and subnetting. These terms are rarely used in the industry anymore because
of the introduction of classless interdomain routing (CIDR).
Given an IP address, its class can be determined from the three high−order bits. Figure 1 shows the
significance in the three high order bits and the range of addresses that fall into each class. For informational
purposes, Class D and Class E addresses are also shown.
In a Class A address, the first octet is the network portion, so the Class A example in Figure 1 has a major
network address of 10. Octets 2, 3, and 4 (the next 24 bits) are for the network manager to divide into subnets
and hosts as he/she sees fit. Class A addresses are used for networks that have more than 65,536 hosts
(actually, up to 16777214 hosts!).
In a Class B address, the first two octets are the network portion, so the Class B example in Figure 1 has a
major network address of 172.16. Octets 3 and 4 (16 bits) are for local subnets and hosts. Class B addresses
are used for networks that have between 256 and 65534 hosts.
In a Class C address, the first three octets are the network portion. The Class C example in Figure 1 has a
major network address of 193.18.9. Octet 4 (8 bits) is for local subnets and hosts − perfect for networks with
less than 254 hosts.
binary bits which can be divisible into a network portion and host portion with the help of a subnet mask. The
32 binary bits are broken into four octets (1 octet = 8 bits). Each octet is converted to decimal and separated
by a period (dot). For this reason, an IP address is said to be expressed in dotted decimal format (for example,
172.16.81.100). The value in each octet ranges from 0 to 255 decimal, or 00000000 − 11111111 binary.
Here is how binary octets convert to decimal: The right most bit, or least significant bit, of an octet holds a
value of 20. The bit just to the left of that holds a value of 21. This continues until the left−most bit, or most
significant bit, which holds a value of 27. So if all binary bits are a one, the decimal equivalent would be 255
as shown here:
1 1 1 1 1 1 1 1
128 64 32 16 8 4 2 1 (128+64+32+16+8+4+2+1=255)
Here is a sample octet conversion when not all of the bits are set to 1.
0 1 0 0 0 0 0 1
0 64 0 0 0 0 0 1 (0+64+0+0+0+0+0+1=65)
And this is sample shows an IP address represented in both binary and decimal.
10. 1. 23. 19 (decimal)
00001010.00000001.00010111.00010011 (binary)
These octets are broken down to provide an addressing scheme that can accommodate large and small
networks. There are five different classes of networks, A to E. This document focuses on addressing classes A
to C, since classes D and E are reserved and discussion of them is beyond the scope of this document.
Note: Also note that the terms "Class A, Class B" and so on are used in this document to help facilitate the
understanding of IP addressing and subnetting. These terms are rarely used in the industry anymore because
of the introduction of classless interdomain routing (CIDR).
Given an IP address, its class can be determined from the three high−order bits. Figure 1 shows the
significance in the three high order bits and the range of addresses that fall into each class. For informational
purposes, Class D and Class E addresses are also shown.
In a Class A address, the first octet is the network portion, so the Class A example in Figure 1 has a major
network address of 10. Octets 2, 3, and 4 (the next 24 bits) are for the network manager to divide into subnets
and hosts as he/she sees fit. Class A addresses are used for networks that have more than 65,536 hosts
(actually, up to 16777214 hosts!).
In a Class B address, the first two octets are the network portion, so the Class B example in Figure 1 has a
major network address of 172.16. Octets 3 and 4 (16 bits) are for local subnets and hosts. Class B addresses
are used for networks that have between 256 and 65534 hosts.
In a Class C address, the first three octets are the network portion. The Class C example in Figure 1 has a
major network address of 193.18.9. Octet 4 (8 bits) is for local subnets and hosts − perfect for networks with
less than 254 hosts.
