This table is called the
routing table and routers use routing protocols to tell each other about the
networks they know of. As networks increase, so do the number of entries in a
routing table. Large routing tables cause increased processing and lower
response time in a router. To reduce the size of routing tables, networks can
be grouped together or summarized using a mask that incorporates them
all. For example, in figure 2-5, a 192.168.10.0/24 subnet has been
divided into smaller subnets of /27 mask. All of these networks connect to
RouterA which it turn is advertising these routes to RouterB. Without
summarization, RouterB will come to know of 8 networks which are available via
RouterA. Since these networks are contagious subnets can have been subnetted
from a /24 address, they can be summarized back into 192.168.1.0/24 network by
RouterA while advertising to RouterB. This way, RouterB comes to know of one
large /24 network only instead of 8 smaller /27 networks.
Figure Summarization
Summarization is similar to
VLSM but in the opposite direction. When using VLSM you move to the right in
terms of the bits (/24 to /25, /25 to /26, so on and so forth) while during
summarization you move to the left (example /27 to /24).
Summarization is somewhat
simple if you remember the following:
- You can only summarize in the block sizes you learned about in VLSM – 128,64,32,16,8,4.
- The network address used for the summarized address is the first network address in the block.
For example, if you want to
summarize networks 192.168.8.0 through 192.168.15.0, first find the block size
you can use. There are 8 networks so the block size of 8 can be used. The first
network address in the block is 192.168.8.0. Now to find the mask of the
summarized route, remember the mask used for a block of 8 – 248. You can also
deduct the block size from 256 to find the mask. Since we are summarizing the
third octet the subnet mask for the summary address will be 255.255.248.0.
Take another example,
172.16.0.0 through 172.16.35.0. This one is not as simple as the first one.
Notice that you have 36 networks to summarize which does not conform to the
block sizes. There are two things that you can do here:
- Summarize in block size of 32 (mask of 224). This will give you a summary address of 172.16.0.0 255.255.224.0 but will only summarize networks 172.16.0.0 through 172.16.31.0. The rest of the 4 networks will be advertised as individual routes.
- Summarize in block of 64 (mask of 192). This will give you a summary address of 172.16.0.0 255.255.192.0 but will summarize networks 172.16.0.0 through 172.16.63.0.
The correct answer depends
on the network. If you are planning to add networks 36 to 63 then the second
options works. Otherwise the first option is the best one.
Take a third example where
you know the summary address of 172.10.16.0 with a mask of 255.255.224.0 and
need to find which networks are being summarized. This is really easy. The
third octet is the interesting octet and gives a block size of 32. This means
the networks 172.10.16.0 through 172.10.47.0 have been summarized.
As a final example,
consider the following networks:
- 192.168.1.0/25
- 192.168.1.128/25
- 192.168.2.0/24
- 192.168.3.0/24
- 192.168.4.0/26
- 192.168.4.64/26
- 192.168.4.128/26
- 192.168.4.192/26
Try to figure out the
summary address that can be used for these networks. If you look carefully the
third octet forms a contiguous block of 4 and can be summarized with the
address 192.168.1.0 255.255.252.0 or 192.168.1.0/22.
In the last example notice
that we summarized a contiguous block of class C using a mask. This is calledsupernetting. Supernetting
is an extension of VLSM and summarization. In summarization you summarize
networks subnetted while in supernetting you summarize a block of contiguous
blocks of Class A, B or C networks. Supernetting is usually practiced by ISPs
to reduce the Internet routing table size.