One such issue involves what to do when there are no more unassigned IP addresses in a subnet.
One alternative solution is to change the mask used on that subnet, making the existing subnet
larger. However, changing the mask could cause an overlap. For example, if 10.1.4.0/24 is
running out of addresses and you make a change to mask 255.255.254.0 (9 host bits, 23
network/subnet bits), an overlap can occur. 10.1.4.0/23 includes addresses 10.1.4.0 to
10.1.5.255; this is indeed an overlap with subnet 10.1.5.0/24. If subnet 10.1.5.0/24 already
exists, using 10.1.4.0/23 would not work. Another alternative for continued growth is to place
all the existing addresses in the mostly full subnet in another larger subnet. There must be a
valid subnet number that is unassigned, that does not create an overlap, and that is larger than
the old subnet. However, this solution causes administrative effort to change the IP addresses.
In either case, both solutions that do not use secondary addressing imply a strategy of using
different masks in different parts of the network. Use of these different masks is called variable-
length subnet masking (VLSM), which brings up another set of complex routing protocol
issues.
uses multiple subnets on the same data link. Secondary IP addressing is simple in concept.
Because more than one subnet is used on the same medium, the router needs to have more than
one IP address on the interface attached to that medium. For example, Figure 5-27 has subnet
10.1.2.0/24; assume that the subnet has all IP addresses assigned. Assuming secondary
addressing to be the chosen solution, subnet 10.1.7.0/24 could also be used on the same
Ethernet. Example 5-8 shows the configuration for secondary IP addressing on Yosemite.