Subnetting
155
Subnets? 62.
Hosts? 2.
Valid subnets? 4, 8, 12, etc., all the way to 248.
Broadcast address for each subnet? (always the number right before
the next subnet)
Valid hosts? (the numbers between the subnet number and the broad-
cast address)
The following table shows you the subnet, valid host, and broadcast
address of the first three and last three subnets in the 255.255.255.252 Class
C subnet.
Practice Example #6C: 255.255.255.128 (/25)
I know I told you that using only one subnet bit was considered illegal in the
original RFCs and that you ought not to do that. But aren't most rules meant
to be broken? This mask can be used when you need two subnets, each with
126 hosts. But our trusty big five questions won't work with this one--it's
special--so I'll just explain it to you. First, use the global configuration com-
mand ip subnet-zero to tell your router to break the rules and use a 1-bit sub-
net mask (this is a default command on all routers running the 12.x Cisco IOS).
Subnet
4
8
12
...
240
244
248
First host
5
9
13
...
241
245
249
Last host
6
10
14
...
242
246
250
Broadcast
7
11
15
...
243
247
251
Should we really use this mask that provides only two hosts?
Yes, this is a very helpful mask in wide area network (WAN) networks. Think
about this as if you had dozens of WAN links connecting to your corporate
office. If you use the 255.255.255.0 mask, then each network would have
254 hosts, but you only use two addresses! That is a waste of 252 hosts per
subnet. If you use the 255.255.255.252 mask, then each subnet has only two
hosts and you don't waste precious addresses.
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