Subnet number question

I was going through the 45-page "Everything you ever wanted to know about IP addressing" and ran across something which I'm stumped about. Whenever I subnet, I always write the proper subnet mask depending on what class(A,B,C). If it was an A network or /8 I'd use 255.0.0.0, if B I'd use 255.255.0.0, if C 255.255.255.0 and so on. Pretty simple, right?

On page 7 they talk about Extended-network-prefix and how if you want to use the third octet of an address to represent the network number you need a mask of 255.255.255.0. The example they give uses a 130.5.5.25 address, so what's going on? 130 would be a Class B from what I have learned. o_O

KG

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Comments

KGhaleon

Nevermind, it seems I misread. They are saying if you wanted to use the entire third octet for the subnet number, then the mask would be 255.255.255.0, which is true.

Nevermind

milliamp

When I complain that classful IP addressing needs to be dropped from modern network exams this is why.

Writers just casually flow from classful to classless in the same breath without ever really including a proper disclaimer or giving clarity.

The alphabet soup A,B,C's are no longer used on most productions networks.

Some routing protocols (RIPv1, IGRP) still use them, but most everything else (RIPv2, EIGRP, OSPF etc.) now use classless addresses, or CIDR.

CIDR addresses are not limited to the default subnet masks of A,B,C etc and RFC 1519 for CIDR was published in 1993

I have been working with computers for over 5 years and classful addresses were no longer being used long before I learned how to "click the blue E".

I didn't even have to know CIDR exists for the network+ (imagine that!), but I actually got a "real" subnetting question on the Linux+ exam, it made me smile