Simplified IP Addressing
Let's say you are granted a full Class C suite of addresses, e.g., 220.127.116.11 as your network address. You are allowed to assign the host addresses (the last byte) as you please. If you use the default Class C netmask of 255.255.255.0 (see Table 5), you can assign host addresses of 18.104.22.168 through 22.214.171.124 on a single network. That's feasible of course, but you may want to break this up into multiple networks of perhaps 25 hosts each.
Let's do some mathematics. If we have 4 bits for hosts, will it be enough? 2<+>4<+>-2 = 14 and is not enough. So, let's use 5 bits for hosts: 2<+>5<+>-2 = 30 which will work. However, we have 8 bits in the last byte for hosts, so let's borrow three bits for subnetworks; then we still have the requisite 5 bits for hosts. Great, but how many subnets do we have? How about 2<+>3<+> = 8? We have, then, eight subnetworks with 30 host addresses on each. If you are doing the math, you are probably saying, “but 8x30 is only 240 addresses; what happened to the others?” Valid question! Oops, don't get sore, but it's time to construct another table. Note that each address will have the form of 210.168.94.last byte, and the SNM (sub-netmask) will have the form 255.255.255.last byte. Let's just work with the last byte.
From Table 2 (or Table 1), we see the SNM will be 255.255.255.224. The 224 comes from the last byte being 11100000. So what are the subnets? Table 6 shows them (last byte only).
Let's detail a few. First, take the smallest. The full subnetwork address of the smallest is 126.96.36.199. The next one up is 188.8.131.52, and so on. Remember that with three bits to work with, we get 2<+>3<+> = 8 subnets, and looking at Table 6, you see them.
Back to the question of why we get only 240 host addresses. “(Gasp)—another table!” Looking at the last byte, we get Table 7.
Now let's answer the question of what happened to the other addresses. To do this, tally all the “invalid addresses”, i.e., those that can't be used for host addresses.
First, we have eight subnets, each with a subnetwork address and a broadcast address. So we lose 8*2 = 16 addresses here. Now if we subtract these 16 from 256, we get 240 available host addresses.
Doing it the other way is much easier. We have eight subnetworks, each with 30 valid IP addresses; this gives us 8*30=240 valid IP addresses total, the magic number.
For fun, let's do one more thing: analyze the sixth subnetwork in a little more detail. The last byte is 10100000 binary or 160 decimal. The full subnet address is 184.108.40.206 decimal, and we use an SNM of 255.255.255.224. Remember, I said to take the subnet address, set all the host bits to 1s and add them to get the broadcast address. If we do this correctly, it should give the same result as Table 7.
We use five bits for host addresses, so the decimal value of the sixth bit is 32. Subtracting 1 gives 31. Thus, setting the five host bits to 1s, i.e., 00011111, gives a value of 31 decimal. Adding this to the last byte of the subnet address (160) gives 191 for the broadcast address, agreeing with Table 7. Here is the “whole Megillah”:
220.127.116.11 The Sub-Network address18.104.22.168-190 Valid host addresses22.214.171.124 Directed Broadcast address
One final point. Some authors use the term “sub-netmask” even when referring to the default netmasks—they are being just a tad loose with their terms. Happy IP addressing, and remember, Linux is inevitable.
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