Chapter 13. Network Administration246 PACSystems* RX7i & RX3i TCP/IP Ethernet Communications User Manual GFK-2224QSubnet Addressing and Subnet MasksSubnet addressing is an extension of the IP address scheme that allows a site to use a single netid for multiplephysical networks. Routing outside the site continues as usual by dividing the IP address into a netid and ahostid 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 00000001netid bits hostid bits(binary)Figure 102: Class B Network netid and hostid Bit FormatsInside 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 resultis shown in the figure below. Selecting the subnet mask shown below would add two additional netid bitsallowing for four physical networks addressed as 0, 64, 128, and 192. The added subnet bits are normally takenfrom the hostid bits adjacent to the netid and the subnet mask identifies these bits.11111111 11111111 11000000 00000000 = 255.255.192.0netid bits hostid bits subnet mask(dotted decimal)(binary)Figure 103: Use of Subnet MaskThe bits in the subnet mask correspond one to one with the Internet address. The bits in the mask that are 1treat the corresponding bits in the IP address as part of the netid bits. The bits in the mask that are 0 treat thecorresponding 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, orsubnetid. Each unique combination of bits in the part of the hostid where subnet mask bits are 1 specifies adifferent physical network.
