Subnet Addressing And Subnet Masks - GE RX3i User Manual

Chapter 13. Network Administration

Subnet Addressing and Subnet Masks

Subnet addressing is an extension of the IP address scheme that allows a site to use a single netid for multiple
physical networks. Routing outside the site continues as usual by dividing the IP address into a netid and a
hostid via the class.
The standard format for the netid bits and hostid bits for an IP address in a Class B network is shown below.
10000000 00000011 00000000 00000001
hostid bits
netid bits
(binary)
Figure 102: Class B Network netid and hostid Bit Formats
Inside a site the subnet mask is used to re-divide the IP address into a custom netid portion and hostid portion.
Consider adding another physical network to Network 2 (a Class B network) in the previous example. The result
is shown in the figure below. Selecting the subnet mask shown below would add two additional netid bits
allowing for four physical networks addressed as 0, 64, 128, and 192. The added subnet bits are normally taken
from the hostid bits adjacent to the netid and the subnet mask identifies these bits.
11111111 11111111 11000000 00000000 = 255.255.192.0
hostid bits
netid bits
subnet mask
(binary)
(dotted decimal)
Figure 103: Use of Subnet Mask
The bits in the subnet mask correspond one to one with the Internet address. The bits in the mask that are 1
treat the corresponding bits in the IP address as part of the netid bits. The bits in the mask that are 0 treat the
corresponding bits as part of the hostid bits.
In effect, two bits of the Class B hostid have been used to extend the netid, creating an extended netid, or
subnetid. Each unique combination of bits in the part of the hostid where subnet mask bits are 1 specifies a
different physical network.
246 PACSystems* RX7i & RX3i TCP/IP Ethernet Communications User Manual GFK-2224Q