162 Chapter 5: IP Addressing
Here are the requirements for a variable-length subnetted mask:
·
The routing protocol must be classless.
·
It must carry the subnet mask in its updates.
·
Summarized addresses must have the same high-order bits.
·
Subnets can be used to address hosts or can be used for further subnetting.
Secondary Addressing: Several Subnets
Multiple IP addresses can be configured per interface. Secondary IP addresses can be used in a
variety of situations. Use secondary IP addresses when:
·
There might not be enough host addresses for a particular network segment. If subnetting
allows up to 254 hosts per logical subnet, but on one physical subnet there are 300 host
addresses, using secondary IP addresses on the routers or access servers would allow two
logical subnets on the same physical subnet.
·
There is a need to support discontiguous subnets. Two subnets of a single network might
be separated by another network. A secondary network can act as a tunnel between two
portions of the network that are not connected.
Figure 5-7 displays a network that uses secondary subnets.
Network
Available Addresses
Network
Available Addresses
116119
Local address: 116
Broadcast address: 119
117, 118
244247
Local address: 244
Broadcast address: 247
245, 246
120123
Local address: 120
Broadcast address: 123
121, 122
248251
Local address: 248
Broadcast address: 251
249, 250
124127
Local address: 124
Broadcast address: 127
125, 126
252255
Local address: 252
Broadcast address: 255
253, 254
Table 5-8
Mask 252: The Serial Mask (Continued)
87200333.book Page 162 Wednesday, August 22, 2001 2:37 PM