Class B Subnetting

Topics: Computer Network - IP Address

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180.55.0.0 will be used for exemplification.

Borrowing less than one octet §

When we borrow less than one octet, the process is mostly the same to Class C Subnetting.

Let’s say we borrow 4 bits from the host portion, such that we build 180.55.0.0/20:

That gives us $n=4$ and $m=12$, so:

• $2^4-2=14$ subnetworks
• $2^{12}-2=4094$ hosts

The change factor is $2^4=16$. With that, we can build the network table:

Subnetwork	Subnetwork IP	First Host	Last Host	Broadcast
0	180.55.0.0	180.55.0.1	180.55.15.254	180.55.15.255
1	180.55.16.0	180.55.16.1	180.55.31.254	180.55.31.255
2	180.55.32.0	180.55.32.1	180.55.47.254	180.55.47.255
3	180.55.48.0	180.55.48.1	180.55.63.254	180.55.63.255
4	180.55.64.0	180.55.64.1	180.55.79.254	180.55.79.255
5	180.55.80.0	180.55.80.1	180.55.95.254	180.55.95.255
6	180.55.96.0	180.55.96.1	180.55.111.254	180.55.111.255
7	180.55.112.0	180.55.112.1	180.55.127.254	180.55.127.255
8	180.55.128.0	180.55.128.1	180.55.143.254	180.55.143.255
9	180.55.144.0	180.55.144.1	180.55.159.254	180.55.159.255
10	180.55.160.0	180.55.160.1	180.55.175.254	180.55.175.255
11	180.55.176.0	180.55.176.1	180.55.191.254	180.55.191.255
12	180.55.192.0	180.55.192.1	180.55.207.254	180.55.207.255
13	180.55.208.0	180.55.208.1	180.55.223.254	180.55.223.255
14	180.55.224.0	180.55.224.1	180.55.239.254	180.55.239.255
15	180.55.240.0	180.55.240.1	180.55.255.254	180.55.255.255

We change more than one octet

Notice how, unlike class C subnetting, with class B subnetting we change the values of the two least significant octets, instead of just one. This is why we can have so many hosts per subnetwork.

An address and a gateway for each subnetwork

Remember: we use the subnetwork IP to identify a whole subnetwork (the whole group of devices under it). It’s good practice to use the first host address for the gateway.

Finally, the subnet mask for this subnet is 255.255.240.0.

Borrowing one octet §

The process for borrowing one octet is easy and straightforward.

Since we borrow one octet, we will build 180.55.0.0/24:

That gives us $n=8$ and $m=8$, so:

• $2^8-2=254$ subnetworks
• $2^8-2=254$ hosts

That gives us a change factor of $1$. We have too many subnetworks to feasibly build a complete table, but we can determine the addresses for a given $n$subnetwork easily enough:

Subnetwork	Subnetwork IP	First Host	Last Host	Broadcast
10	180.55.10.0	180.55.10.1	180.55.10.254	180.55.10.255
82	180.55.82.0	180.55.82.1	180.55.82.254	180.55.82.255
101	180.55.101.0	180.55.101.1	180.55.101.254	180.55.01
128	180.55.128.0	180.55.128.1	180.55.128.254	180.55.128.255
200	180.55.200.0	180.55.200.1	180.55.200.254	180.55.200.255

The corresponding subnet mask is 255.255.255.0.

Borrowing more than one octet §

When we borrow more than one octet, the process is a bit more complicated.

Let’s say we borrow 11 bits, such that we build 180.55.0.0/27:

That gives us $n=11$ and $m=5$, so:

• $2^{11}-2=2046$ subnetworks
• $2^5-2=30$ hosts

We have no change factor this time around. Instead, we will follow an algorithm to determine the address.

Notice how, below the table (in red), we have the values corresponding to each bit relative to the octet they’re in, while above the table (in orange) we have the values corresponding to each bit relative to the total amount of bits we’re borrowing.

To get the address for a given $n$ subnetwork, we take the values from below such that they sum $n$. Then, for each octet, we take the corresponding bit values and add them. The result will be the value that will go to the IP address for that specific octet.

For example, for the 10th subnetwork, we take the values $2^3+2^1=8+2=10$. The corresponding bit values relative to the octets are $1$ (for the second octet) and $64$ (for the first octet). Thus, we get the address 180.55.1.64.

We have way too many subnetworks to feasibly build a complete table, but we can determine the addresses for a few subnetworks:

Subnet	Subnet IP	First Host	Last Host	Broadcast
10	180.55.1.64	180.55.1.65	180.55.1.94	180.55.1.95
11	180.55.1.96	180.55.1.97	180.55.1.116	180.55.117
20	180.55.2.128	180.55.2.129	180.55.2.158	180.55.2.159
21	180.55.2.160	180.55.2.161	180.55.2.180	180.55.2.181
128	180.55.16.0	180.55.16.1	180.55.16.30	180.55.16.31
260	180.55.32.128	180.55.32.129	180.59.32.158	180.59.32.159
1000	180.55.125.0	180.55.125.1	180.55.125.30	180.55.125.31
2046	180.55.255.224	180.55.255.225	180.55.255.254	180.55.255.255

The corresponding subnet mask is 255.255.255.224.