HP-UX LAN Administrator's Guide (Feburary 2007)

Network AddressingDRAFT COPY
Variable-Length Subnet Addressing
Chapter 7120
This will allow for more growth bits between the subnet field and the host fields as shown
below.
Figure 7-11 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 7-11, 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 7-12, 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.
0s
1s & 0s
1s & 0s
subnet field host field
Both hosts and subnets
can grow here.