GE PACSystems RX7i, User Manual

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Chapter 13. Network Administration
246 PACSystems* RX7i & RX3i TCP/IP Ethernet Communications User Manual GFK-2224Q
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
netid bits
hostid 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
11 000000 00000000 = 255.255. 192. 0
netid bits
hostid bits
subnet mask
(dotted decimal)
(binary)
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.