means that smaller subnets can be combined into a
larger network, and instead of having several routing
table entries for each individual subnet, a single routing
table entry can encompass them all. For example, when
an ISP assigns a certain prefix range to one of its
customers, the ISP does not need to know all the details
of how the customer is implementing subnetting in its
internal network. All the ISP needs to know is that the
whole prefix is assigned to the customer, and it can be
reached via the customer’s border routers. Supernetting
is critical in maintaining an Internet routing table of a
manageable size.
Another important aspect when building a routing table
is the longest prefix match concept. Longest prefix match
means that when the router is looking in the routing
table for a match to route the data packet toward its
destination, it does so by trying to match it to the longest
prefix it has in the table. For example, let’s assume that
the routing table has two entries: 10.10.10.0/24 via
interface GigabitEthernet0/0 and 10.10.0.0/16 via
interface GigabitEthernet0/1. If the router receives a
data packet destined for 10.10.10.15, it looks in its
routing table, and it will match the longest-prefix
network (in this case, 10.10.10.0/24) and route the
packet out its GigabitEthernet0/0 interface. If the data
packet has destination address 10.10.20.15, the router
matches the other entry, 10.10.0.0/16, since that is the
longest prefix match it has for that subnet.
EXAM PREPARATION TASKS
As mentioned in the section “How to Use This Book” in
the Introduction, you have a couple of choices for exam
preparation: the exercises here, Chapter 19, “Final
Preparation,” and the exam simulation questions on the
companion website.