User guide

Internetworking Primer A-25
Desktop Protocols
Design and administration of an IP network requires an understanding of IP subnetting.
Review the document Beginning IP for New Users which is available on CIO to ensure
basic familiarity with the concepts of IP subnetting.
IP Network Numbering
The IP address space is 32 bits long and is used to represent both the network and the node.
An IP address is written as four decimal numbers separated by dots (called dotted decimal
notation); for example, 172.16.10.1. Each of the four numbers is called an octet because it
represents 8 bits, and each octet has a maximum value of 255. For routing purposes, the IP
address is broken into three parts: major network, subnet, and node. (See Figure A-10.)
Figure A-10 IP Address Broken into Network, Subnet, and Node Fields
Major networks are allocated by the InterNIC to identify different internetworks that are
connecting to the Internet. Subnets are used to number each data link (network) within an
internetwork, and the bits to the right of the subnet field are used for the node address.
In Figure A-11, the major network is the class B network, 172.16.0.0. The subnet mask is
255.255.255.0. Networks are uniquely numbered at the third octet. In Figure A-11, subnets
1, 2, 10, 20, 30, and 40 have been assigned to the distinct networks.
H3547
172.16 10 1
Network Subnet Node