Subnetting a Class B Network
Subnetting a Class B
network address
Class B subnets
Class C network addresses only have eight bits to manipulate into
subnets. However, a Class B has 16 bits to play with. This will allow more
subnets with more hosts per subnet than a Class C network ever could.
Table 1 lists all of the possible Class B subnets:
Table 1 |
Mask |
Binary |
Subnets |
Hosts per subnet |
255.255.128.0 |
10000000.00000000 |
2 |
32,766 |
255.255.192.0 |
11000000.00000000 |
2 |
16,382 |
255.255.224.0 |
11100000.00000000 |
6 |
8,190 |
255.255.240.0 |
11110000.00000000 |
14 |
4,094 |
255.255.248.0 |
11111000.00000000 |
30 |
2,046 |
255.255.252.0 |
11111100.00000000 |
62 |
1,022 |
255.255.254.0 |
11111110.00000000 |
126 |
510 |
255.255.255.0 |
11111111.00000000 |
254 |
254 |
255.255.255.128 |
11111111.10000000 |
510 |
126 |
255.255.255.192 |
11111111.11000000 |
1022 |
62 |
255.255.255.224 |
11111111.11100000 |
2,046 |
30 |
255.255.255.240 |
11111111.11110000 |
4,094 |
14 |
255.255.255.248 |
11111111.11111000 |
8,190 |
6 |
255.255.255.252 |
11111111.11111100 |
16,382 |
2 |
All possible
Class B subnets
There are quite a few more masks we can use with a Class B network address
than we can with a Class C network address. Remember that this is not harder
than subnetting with Class C, but it can get confusing if you don't pay
attention to where the subnet bits and host bits are in a mask. This takes
practice!
We'll start with the Class B subnet mask of 255.255.192.0 and figure out the
subnets, broadcast address, and valid host range. We will answer the same
five questions we answered for the Class C subnet masks:
- How many subnets does this mask provide?
- How many hosts per subnet does this mask provide?
- What are the valid subnets?
- What is the broadcast address for each subnet?
- What is the host range of each subnet?
Before we answer these questions, there is one difference you need to be
aware of when subnetting a Class B network address. When subnetting in the
third octet, you need to add the fourth octet. For example, on the
255.255.192.0 mask, the subnetting will be done in the third octet. To
create a valid subnet, you must add the fourth octet of all 0s and all 1s
for the network and broadcast address (0 for all 0s and 255 for all 1s).
Example 1: Answers for the
255.255.192.0 mask
- 2-2=2 subnets
- 2-2=16,382 hosts per subnet
- 256-192=64.0, 128.0
- Broadcast for the 64.0 subnet is 127.255.
Broadcast for the 128.0 subnet is 191.255.
- The valid hosts are:
Subnet |
64.0 |
128.0 |
first host |
64.1 |
128.1 |
last host |
127.254 |
191.254 |
broadcast |
127.255 |
191.255 |
Notice that the numbers in the third octet are the same numbers we used in
the fourth octet when subnetting the 192 mask. The only difference is that
we add 0 and 255 in the fourth octet.
For the 64.0 subnet, all the hosts between 64.1 and 127.254 are in the 64
subnet. In the 128.0 subnet, the hosts are 128.1 through 191.254.
I know this is confusing, but I promise if you read this article carefully,
I can make it easier for you.
Work through a few more with me, and it should start to become clearer.
Example 2: 255.255.240.0
- 2-2=14 subnets
- 2-2=4094 hosts per subnet
- 256-240=16.0, 32.0, 48.0, 64.0, etc.
- Broadcast for the 16.0 subnet is 31.255. Broadcast
for the 32.0 subnet is 47.255, etc.
- The valid hosts are:
Subnet |
16.0 |
32.0 |
48.0 |
64.0 |
first host |
16.1 |
32.1 |
48.1 |
64.1 |
last host |
31.254 |
47.254 |
63.254 |
79.254 |
broadcast |
31.255 |
47.255 |
63.255 |
79.255 |
Example 3: 255.255.248.0
- 2-2=30 subnets
- 2-2=2,046 hosts per subnet
- 256-248=8.0, 16.0, 24.0, 32.0, 40.0, 48.0, 56.0,
64.0, etc.
- Broadcast for the 8.0 subnet is 15.255. Broadcast
for the 16.0 subnet is 23.255, etc.
- The valid hosts are:
Subnet |
8.0 |
16.0 |
24.0 |
32.0 |
40.0 |
48.0 |
56.0 |
64.0 |
first host |
8.1 |
16.1 |
24.1 |
32.1 |
40.1 |
48.1 |
56.1 |
64.1 |
last host |
15.254 |
23.254 |
31.254 |
39.254 |
47.254 |
55.254 |
63.254 |
71.254 |
broadcast |
15.255 |
23.255 |
31.255 |
39.255 |
47.255 |
55.255 |
63.255 |
71.255 |
Example 4: 255.255.252.0
- 2-2=62 subnets
- 2-2=1,022 hosts per subnet
- 256-252=4.0, 8.0, 12.0, 16.0, 20.0, 24.0, 28.0,
32.0, etc.
- Broadcast for the 4.0 subnet is 7.255. Broadcast
for the 8.0 subnet is 11.255, etc.
- The valid hosts are:
Subnet |
4.0 |
8.0 |
12.0 |
16.0 |
20.0 |
24.0 |
28.0 |
32.0 |
first host |
4.1 |
8.1 |
12.1 |
16.1 |
20.1 |
24.1 |
28.1 |
32.1 |
last host |
7.254 |
11.254 |
15.254 |
19.254 |
23.254 |
27.254 |
31.254 |
35.254 |
broadcast |
7.255 |
11.255 |
15.255 |
19.255 |
23.255 |
27.255 |
31.255 |
35.255 |
Example 5: 255.255.255.0
- 2-2=254 subnets
- 2-2=254 hosts per subnet
- 256-255=1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0,
etc.
- Broadcast for the 1.0 subnet is 1.255. Broadcast
for the 2.0 subnet is 2.255, etc.
- The valid hosts are:
Subnet |
1.0 |
2.0 |
3.0 |
4.0 |
5.0 |
6.0 |
7.0 |
8.0 |
first host |
1.1 |
2.1 |
3.1 |
4.1 |
5.1 |
6.1 |
7.1 |
8.1 |
last host |
1.254 |
21.254 |
3.254 |
4.254 |
5.254 |
6.254 |
7.254 |
8.254 |
broadcast |
1.255 |
2.255 |
3.255 |
4.255 |
5.255 |
6.255 |
7.255 |
8.255 |
That last example was pretty simple. I hope you notice a pattern now. All
the numbers were basically the same except we added the fourth octet into
the address.
The more difficult process of subnetting a Class B network address is when
you start using bits in the fourth octet for subnetting. For example, what
happens when you use this mask with a Class B network address:
255.255.255.128? Is that valid? Absolutely! There are nine bits for
subnetting and seven bits for hosts. That is 510 subnets, each with 126
hosts. However, it is the most difficult mask to figure out the valid hosts
for.
Example 6: The Class B 255.255.255.128 subnet mask:
- 2-2=510 subnets
- 2-2=126 hosts per subnet
- For the third octet, the mask would be 256-255=1,
2, 3, 4, 5, 6, etc.
- For the fourth octet, the mask would be
256-128=128, which is one subnet if it is used. However, if you turn the
subnet bit off, the value is 0. This means that for every subnet in the
third octet, the fourth octet has two subnets: 0 and 128, for example
1.0 and 1.128.
- Broadcast for the 0.128 subnet is 128.255; the
broadcast for the 1.0 subnet is 1.127. Broadcast for the 1.128 subnet is
1.255, etc.
- The valid hosts are:
Subnet |
0.128 |
1.0 |
1.128 |
2.0 |
2.128 |
3.0 |
3.128 |
4.0 |
first host |
0.129 |
1.1 |
1.129 |
2.1 |
2.129 |
3.1 |
3.129 |
4.1 |
last host |
0.254 |
1.126 |
1.254 |
2.126 |
2.254 |
3.126 |
3.254 |
4.126 |
broadcast |
0.255 |
1.127 |
1.255 |
2.127 |
2.255 |
3.127 |
3.255 |
4.127 |
The thing to remember is that for every subnet in the third octet, there are
two in the fourth octet: 0 and 128. For the 0 subnet, the broadcast address
is always 127. For the 128 subnet, the broadcast address is always 255.
Let's continue with more subnetting into the fourth octet. This is exactly
like subnetting a Class C network address, but the third octet is part of
the subnet address.
Example 7: Class B network
255.255.255.192
- 2-2=1022 subnets
- 2-2=62 hosts per subnet
- 256-255=1.0, 2.0, 3.0, etc. for the third octet.
256-192=64, 128, 192 for the fourth octet. For every valid subnet in the
third octet, we get four subnets in the fourth octet: 0, 64, 128, and
192.
- Broadcast for the 1.0 subnet is 1.63, since the
next subnet is 1.64. Broadcast for the 1.64 subnet is 1.127, since the
next subnet is 1.128. Broadcast for the 1.128 subnet is 1.191, since the
next subnet is 1.192. Broadcast for the 1.192 subnet is 1.255.
- The valid hosts are as follows:
Subnet |
0.64 |
0.128 |
0.192 |
1.0 |
1.64 |
1.128 |
1.192 |
2.0 |
first host |
0.65 |
0.129 |
0.193 |
1.1 |
1.65 |
1.129 |
1.193 |
2.1 |
last host |
0.126 |
0.190 |
0.254 |
1.62 |
1.126 |
1.190 |
1.254 |
2.62 |
broadcast |
0.127 |
0.191 |
0.255 |
1.63 |
1.127 |
1.191 |
1.255 |
2.63 |
On this one, the 0 and 192 subnets are valid, since we are using the third
octet as well. The subnet range is 0.64 through 255.128. 0.0 is not valid
since no subnet bits are on. 255.192 is not valid because then all subnet
bits would be on.
Example 8: Class B network 255.255.255.224
- 2-2=2046 subnets
- 2-2=30 hosts per subnet
- 256-255=1.0, 2.0, 3.0, etc. for the third octet.
256-224=32, 64, 96, 128, 160, 192 for the subnet value. (For every value
in the third octet, we get eight subnets in the fourth octet: 0, 32, 64,
96, 128, 160, 192, 224.)
- Broadcast for the 1.0 subnet is 1.63, since the
next subnet is 1.64. Broadcast for the 1.64 subnet is 1.127, since the
next subnet is 1.128. Broadcast for the 1.128 subnet is 1.191, since the
next subnet is 1.192. Broadcast for the 1.192 subnet is 1.255.
- The valid hosts are:
Subnet |
0.32 |
0.64 |
0.96 |
0.128 |
0.160 |
0.192 |
0.224 |
1.0 |
first host |
0.33 |
0.65 |
0.97 |
0.129 |
0.161 |
0.193 |
0.225 |
1.1 |
last host |
0.62 |
0.94 |
0.126 |
0.158 |
0.190 |
0.222 |
0.254 |
1.30 |
broadcast |
0.63 |
0.95 |
0.127 |
1.159 |
0.191 |
0.223 |
0.255 |
1.31 |
For this subnet mask, the 0 and 224 subnets are valid as long as not all
subnet bits in the third octet are off or all subnet bits in the fourth
octet are on.
When would we use this valuable information? All the time! For example, if
you have a host configuration of 172.16.10.33 255.255.255.224, what subnet,
broadcast address, and valid host range is this host a member of? (We would
solve this question with the information presented above.)
256-224=32, 64
Bingo! In the fourth octet, the host address is 33. That is between 32 and
64, so the host is in the 32 subnet, which has a broadcast address of 63,
and the valid host range is 33-62. Easy. Just remember that the subnet is
10.32 because the third octet is part of the subnet address.
Let's try another one. You have a host configuration of 172.16.10.33
255.255.255.240. What subnet, broadcast address, and valid host range is
this host a member of?
Since we did not go through this mask in this Daily Drill Down, you'll have
to figure it out on your own. It is done the same way as all the others.
256-240=16, 32, 48
Bingo! The host is in the 10.32 subnet, which has a broadcast address of
10.47 and a valid host range of 10.33 through 10.46.
Let's keep going: You have a host configuration of 172.16.10.33
255.255.255.248. What subnet, broadcast address, and valid host range is
this host a member of?
256-248=8, 16, 24, 32, 40
Bingo! The host is in the 10.32 subnet, which has a broadcast address of
10.39 and valid host range of 10.33 through 10.38. Easy, huh?
One more: You have a host configuration of 172.16.10.17 255.255.255.252.
What subnet, broadcast address, and valid host range is this host a member
of?
256-252=4, 8, 12, 16, 20
Bingo! You have a subnet of 10.16, with a broadcast of 10.19 and valid host
range of 10.17 through 10.18.
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