Installing and Administering LAN/9000 Software

156 Chapter6

Network Addressing

Variable-Length Subnet Addressing

Figure 6-12 Mirror Image Subnet and Host Field Allocation

In this case, both the host ﬁeld and subnet ﬁeld have considerably more

growing space than before, although the combined growing space is the

same. As it is difﬁcult to predict how many hosts might end up in a

subnet, or how many subnets there might eventually be, this

arrangement allows for maximum ﬂexibility in growth.

Assigning Variable-Length Subnet Masks

In Figure 6-12, the boundary between the host and subnet ﬁelds is

shown in the middle of the growth area. The boundary, however, could

exist anywhere within the growth area. The subnet mask determines

where the boundary is located. “Ones” in the subnet mask indicate

subnet bits, and “zeros” indicate host bits. To minimize the amount of

rework after the initial planning of your network, you should choose a

subnet mask for a given subnet based on the projected growth of that

subnet. As shown in Figure 6-13, the subnet is projected to have a

maximum size of 14 hosts. Therefore, the subnet mask should be

255.255.255.240. There are two remaining growth bits for this subnet. If

the subnet grows beyond 14 hosts, you may choose to change one of the

two remaining growth bits to a host bit. The new subnet mask will be

255.255.255.224.

Variable-length subnet masks are assigned using the netmask

parameter of the ifconfig(1M) command, SUBNET_MASK in netconf

with a 32-bit subnet mask indicated in dot notation, or SAM. In the

example below with a Class C IP address, the host portion now has three

types of assigned bits: subnet number bits, growth bits, and host bits,

with a chosen subnet mask that allows for growth in both the host ﬁeld

and subnet ﬁeld without a mask change.

1s & 0s

subnet field host field

Both hosts and subnets

can grow here.