Subnetting
161
subnet masks you can play with though. And worse, it actually is a really
good subnet to use in production because it creates over 500 subnets with
126 hosts for each subnet--a nice mixture. So, don't skip over it!
Subnets? 2
9
- 2 = 510.
Hosts? 2
7
- 2 = 126.
Valid subnets? Okay, now for the tricky part. 256
- 255 = 1, 2, 3, etc.,
for the third octet. But you can't forget the one subnet bit used in the
fourth octet. Remember when I showed you how to figure one subnet
bit with a Class C mask? You figure this the same way. (Now you
know why I showed you the 1-bit subnet mask in the Class C section--
to make this part easier.) You actually get two subnets for each fourth
octet value, hence the 510 subnets. For example, if the third octet is
showing subnet 3, the two subnets would actually be 3.0 and 3.128.
Broadcast address for each subnet?
Valid hosts?
The following table shows how you can create subnets, valid hosts, and
broadcast addresses using the Class B 255.255.255.128 subnet mask. (The
first seven subnets are shown, and then the last subnet).
Practice Example #6B: 255.255.255.192 (/26)
This one gets just a little tricky. Both the 0 subnet as well as the 192 subnet
could be valid in the fourth octet. It just depends on what that third octet
is doing.
Subnets? 2
10
- 2 = 1022.
Hosts? 2
6
- 2 = 62.
Valid subnets? 256
- 192 = 64 and 128. And as long as all the subnet
bits on the third are not all off, then subnet 0 in the fourth octet is
valid. Also, as long as all the subnet bits in the third octet are not all
on, 192 is valid in the fourth octet as a subnet.
Subnet
0.128 1.0
1.128 2.0
2.128 3.0
3.128 ...
255.0
First host
0.129 1.1
1.129 2.1
2.129 3.1
3.129 ...
255.1
Last host
0.254 1.126 1.254 2.126 2.254 3.126 3.254 ...
255.126
Broadcast
0.255 1.127 1.255 2.127 2.255 3.127 3.255 ...
255.127
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