Subnetting class A question, WHY -2

Why clas B and C networks dont require -2 to calculate number of subnets, but Class A esquires -2 from the total? I tried to look up for that, but i couldnt find any other resource showing this. I got this info from Odom`s book. I am pretty well with subnetting, but this confused me. I have the ICND1 exam scheduled Nov 11, so i want to get it straight.

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Comments

boredgamelad

This doesn't have to do with subnetting. This has to do with the total number of (classful) networks in each class and which IP addresses are valid to assign.

Class A technically runs from 0.0.0.0 to 127.255.255.255 and therefore there are 128 (2^7) Class A networks (0.0.0.0 through 127.0.0.0), but you can only use 126 of them (2^7 - 2). You're asking yourself why this is. Well, look at the start and end ranges on that table. What are the two ranges Class A is missing and why?

2URGSE

boredgamelad wrote: »

This doesn't have to do with subnetting. This has to do with the total number of (classful) networks in each class and which IP addresses are valid to assign.

Class A technically runs from 0.0.0.0 to 127.255.255.255 and therefore there are 128 (2^7) Class A networks (0.0.0.0 through 127.0.0.0), but you can only use 126 of them (2^7 - 2). You're asking yourself why this is. Well, look at the start and end ranges on that table. What are the two ranges Class A is missing and why?

++++ 1, agreed!!!

Missing loopback 127.0.0.0 and 10.0.0.0 (private networks)

Dynaice

Well the 10.0.0.0 is still a class A network usable in the private. It's just you don't use the all 0.0.0.0 network. Never seen a 0.0.0.0 network other than wildcard mask.

TurK-FX

boredgamelad wrote: »

This doesn't have to do with subnetting. This has to do with the total number of (classful) networks in each class and which IP addresses are valid to assign.

Class A technically runs from 0.0.0.0 to 127.255.255.255 and therefore there are 128 (2^7) Class A networks (0.0.0.0 through 127.0.0.0), but you can only use 126 of them (2^7 - 2). You're asking yourself why this is. Well, look at the start and end ranges on that table. What are the two ranges Class A is missing and why?

Now it makes sense. I figured 0.0.0.0 network but never occurred to me 127 loopback. Thnx for the explanation. I could just memorize it, but this way i will never forget it, cus i got the logic behind it.

mohamedshajid

Absolutely agreed with boredgamelad post actually it's a Class A Network so there is a limitation between them like this 1.0.0.0 to 127.255.255.255 This is a good post to find out some rare and uncommon questions with you