Subnetting Question

[petejnetwork]

Hi All, this is my first post as I have only just joined. I am currently studying for my network plus and am just starting to get to grips with subnetting. I have been practising using Class C addresses and thought all was going well until I tried with Class A and Class B. The problem I have (and I am sure it is pretty basic, but I am struggling a little) is once an octet gets to 255 is the next step 1 or 0 and how does it effect the previous octet? For example If I have a class B IP address of 172.24.140.254, how would this increment one step at a time? Would the next step be 172.24.140.255, or 172.24.141.0 or 172.24.141.1. I am guessing the three of those suggestions are incorrect. If somebody could give me the next three or four steps from 172.24.140.254 it would clear it up for me. Thanks in advance, and sorry if it is a basic question. Pete

 

[ScottMac]

Two points to ALWAYS remember:
If the "host portion" of the address is all ones, it is a broadcast address
If the host portion is all zeros, then it is a "network address"

Should the "host portion" spill into the next octet upstream, the two points above still apply. That is why you can have a "255" in the last octet and -not- be a broadcast, or an address with zero in the last octet is not necessarily a "network address." This tends to show up on tests.

If you're working on a /23 or less (22, 21, 20, etc.), then the bits of the next octet upstream are part of the host address.

SO, for your examples 172.24.140.254 ... the next increment is 255. You need to remember to specify a mask when you give an address in this context, otherwise there is no way to determine if you are at the end of the range (and using a broadcast address).
Assuming you're on a /22 for example, then the next increment after 172.24.140.255 would be 172.24.141.0. and because the most significant bits are not all ones or all zeros, the lower octet bits aren't either a broadcast or network address (even though the lower octet is all one or all zeros ... the ENTIRE range of host bits has to be considered.

 

[petejnetwork]

Thanks Scott

 

[Mushkins]

The best subnetting advice anyone can give: practice, practice, practice.

Every single networking textbook at that level has a chapter on subnetting, and a billion practice examples in the back. Do them all. Do them again. Do them a third time. Keep a subnetting calculator app next to you to check your work if the book doesnt have the solutions.

It's just one of those topics that most people bang their heads against until it finally just "clicks" and it all suddenly makes perfect sense.

 

[lif_andi]

Took Network+ myself a few months back, don't remember them talking about VLSM but I might be mistaken.

To answer your question, although its already been answered, then 172.24.140.255 would be the next step from .254, but 255 is a broadcast address if you have a /24 mask. If you have /16 for your Class B address, then essentially you have ranges from 172.24.0.0 to 172.24.255.255 where the 172.24.0.0 is the network ID and 172.24.255.255 is the broadcast IP, everything inbetween goes for hosts if you want.

If you're studying Variable length subnet masks already, then that is a whole other discussion.

EDIT: The next step in your question is 172.24.140.255 and the one after that would be.0 provided you have a /16 subnet mask.

 

[petejnetwork]

Thanks so much guys

 

[ITJunkie]

It's just one of those topics that most people bang their heads against until it finally just "clicks" and it all suddenly makes perfect sense.

Couldn't have said this better. Can't remember my AHA moment but that's the way it happened...one day it just "clicked" and the rest was gravy.