How Does Sub-Subnetting Work?

How Does Sub-Subnetting Work?

Each subnet address at Cornell is assigned a "subnet mask." A subnet mask defines how many bits are used for the network address and how many for the host address.

The subnet mask address is 255.255.255.0, and it currently is the same for all LANs. If you convert the subnet mask address to its binary form, it looks like this:

Subnet mask: 11111111 11111111 11111111 00000000

If you convert our example host address (128.253.21.58) to its binary form, it looks like this:

Host address: 10000000 11111101 00010101 00111010

Together they look like this:

Subnet mask: 11111111 11111111 11111111 00000000

Host address: 10000000 11111101 00010101 00111010

The subnet mask when shown this way, as an overlay on the host address, essentially tells the computer which part of the IP address is a network address and which part is a host address. Everything in the host address that corresponds to a 1 in the subnet mask is a network address and everything in the host address that corresponds to a 0 in the subnet mask is a host address.
Sub-subnetting

Many LANs at Cornell are sub-subnetted.

Sub-subnetting is based on the same concept as subnetting. With sub-subnetting, the mask will be 255.255.255.192. In binary form, this address looks like this:

Sub-subnet mask: 11111111 11111111 11111111 11000000

When you compare the sub-subnet mask to the example IP host address (128.253.21.58), they look like this:

Sub-subnet mask: 11111111 11111111 11111111 11000000

Host address: 10000000 11111101 00010101 00111010

Like the old subnet mask, the sub-subnet mask tells the computer which part of the IP address is the network address and which part is a host address. As explained above, everything in the host address that corresponds to a "1" in the sub-subnet mask is a network address and everything in the host address that corresponds to a 0 in the sub-subnet mask is a host address.

As you can see, there are now ones (1's) in the last byte of the sub-subnet mask. (This is the part, in the subnet mask, that was all zeros (0's) and that identified the host portion of an address.) The ones (1's) that appear in this byte of the sub-subnet mask identify the first two bits of the last byte of an IP address as part of the network portion of the address. The remaining zeros (0's) identify the host portion of the address. By increasing the number of bits assigned to network addressing, the number of possible network addresses increases and the number of possible host addresses for each LAN decreases.
Why is this important?

Gateways need to forward packets to other gateways to get them to the destination LAN. Each network interface card on the gateway is assigned an IP address and a sub-subnet mask. This enables the gateways to route packets from one LAN to another LAN. Once the packet arrives at a gateway that is attached to the destination LAN, the gateway then uses the two bits of the sub-subnet portion of the IP address (the first two bits of the last byte of the IP address) to decide to which sub-subnetted LAN to send the packet.

In order for this to work, each of the LANs connected to a given gateway must have a different set of host addresses. As you know, the host address you assign, for example 58, is translated into a binary address (00111010). Remember, even though you think of this as a host address, the sub-subnet mask forces the gateway to think of the first two bits as part of the network address. If two LANs attached to the same gateway each have hosts with an address of 58, the first two bits in the binary translation will be the same, and even though the hosts are physically on two separate LANs, the gateway won't know to which LAN to send a packet.

To make sure each LAN has a unique set of host addresses, network administrators can no longer assign host numbers from the entire range. Now, each administrator will be assigned a subset of the host addresses available. Within each subset, broadcast, gateway, and diagnostic addresses must be reserved. So, for example, Administrator X can have server addresses at 6 through 9 and workstation addresses at 10 through 63; Administrator Y can have server addresses at 70 through 73 and workstation addresses 74 through 126, etc. If your current host addresses fall outside the range assigned to your LAN, you need to reassign host addresses on your network.