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OfficeConnect VPN Firewall User’s Manual Chapter 19. IP Addresses, Network Masks, and Subnets

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19 IP Addresses, Network Masks,

and Subnets

19.1 IP Addresses

Note

This section pertains only to IP addresses for IPv4 (version 4 of

the Internet Protocol). IPv6 addresses are not covered.

This section assumes basic knowledge of binary numbers, bits,

and bytes. For details on this subject, see Appendix 18.

IP addresses, the Internet's version of telephone numbers, are used to identify

individual nodes (computers or devices) on the Internet. Every IP address

contains four numbers, each from 0 to 255 and separated by dots (periods), e.g.

20.56.0.211. These numbers are called, from left to right, field1, field2, field3,

and field4.

This style of writing IP addresses as decimal numbers separated by dots is

called dotted decimal notation. The IP address 20.56.0.211 is read "twenty dot

fifty-six dot zero dot two-eleven."

19.1.1 Structure of an IP address

IP addresses have a hierarchical design similar to that of telephone numbers.

For example, a 7-digit telephone number starts with a 3-digit prefix that identifies

a group of thousands of telephone lines, and ends with four digits that identify

one specific line in that group.

Similarly, IP addresses contain two kinds of information.

 Network ID

Identifies a particular network within the Internet or Intranet

 Host ID

Identifies a particular computer or device on the network

The first part of every IP address contains the network ID, and the rest of the

address contains the host ID. The length of the network ID depends on the

network's class (see following section). Table 19.1 shows the structure of an IP

address.

Table 19.1. IP Address structure

Field1

Field2

Field3

Field4

Class A

Network ID

Host ID

Class B

Network ID

Host ID

Class C

Network ID

Host ID

Here are some examples of valid IP addresses:

Class A: 10.30.6.125 (network = 10, host = 30.6.125)

Class B: 129.88.16.49 (network = 129.88, host = 16.49)

Class C: 192.60.201.11 (network = 192.60.201, host = 11)

19.2 Network classes

The three commonly used network classes are A, B, and C. (There is also a

class D but it has a special use beyond the scope of this discussion.) These

classes have different uses and characteristics.

Class A networks are the Internet's largest networks, each with room for over 16

million hosts. Up to 126 of these huge networks can exist, for a total of over 2

1 2 ... 120 121 122 123 124 125 126 127 128 129 130 ... 155 156

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