Subnetting for the Broadcast, Video and Audio Systems Engineer

The last edition of this column talked about subnets and introduced folding paper as an illustration of how subnets can be understood. This will be very useful as we proceed.

MORE IPv4 SUBNETTING BASICS

Consider an analog clock. Start the second hand at 12:00 and follow it clockwise around the dial. At 12:00, 60 seconds have passed. Zero seconds and 60 seconds are the same place on the dial. When the second hand gets to 60, the minute hand has moved one minute. Since we keep track of time in minutes and seconds, we usually think of seconds from zero to 59, then at 60 seconds we re-set our seconds count to zero and add a minute. It's like counting from nine to 10, where we increment the tens column when we pass nine.

It's like that looking at IP and subnet octets. We can go from zero to 255 in each octet, but 256 means the octet is filled. We don't have room for 256, because 255 is the limit, i.e. all binary bits are set to 1. When we hit 256, the octet changes to zero and we increment the octet to the left by one.

At home, your computer might have an IP address like 192.168.1.10. For this address, your subnet mask is probably 255.255.255.0. You decide you want two networks. Doing the mental math, you say Two networks from one means one fold of the paper. Each fold of the paper is a bit borrowed from the host address field. Here, you borrow one bit. That means you need to change your subnet mask. Here's how that works:

You started off with 255.255.255.0. In binary, that's all ones in the first three octets. To borrow, you need the first bit in the fourth octet. Here's how it works:

11111111.11111111.11111111.00000000 is 255.255.255.0. This is where you start. You borrow the first bit in the fourth octet. The new subnet mask is 11111111.11111111.11111111.10000000.

Borrowing Table 9 from an earlier article and revising it to show the borrowing, here's the new fourth octet:

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