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TOPIC: Subnetting Question

Subnetting Question 8 years 5 months ago #26623

  • ap1227
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Hey guys,
I am a new IT student (and a new forum member) and am having my first bout with subnetting. I understand the fundamentals but am still a bit confused when asked to subnet logically. I understand the IP address class types, the amount of subnets, and quantity of hosts that can be found within a subnet. But my real problem is finding some IP address ranges with a Class A or Class B subnet.

Here is my problem:

IP address:

77.123.28.167

Subnet Mask:

255.255.224.0

This is obviously a Class A IP address. I know I can get 2048 subnets and 8190 hosts with this subnet. I am asked to find the first two ranges. Now would the first range be:

77.123.33.0 (using only the third octet "1" bit) or

77.123.32.1 (using the "1" bit in the fourth octet)?



I have read my text book:
Managing a Microsoft Windows Server 2003 Network, Enhanced; Eckert, Schitka
Thompson Course Technology
...and have browsed online looking for an inkling but I have found nothing in relation with this question of possible IP address ranges. I know there can quite a number of ranges.

I am sorry if this is an unclear question. Like I said, I am new to this and am starting pave the way for a new career. Any and all help will be appreciated.

Thank you for your time in reading this.

Adam
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Re: Subnetting Question 8 years 5 months ago #26624

  • Chojin
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You're asked to find the first 2 ranges?!?

You can make a range if you devide your subnet into multiple subnets... then for example if you have a class C subnet


192.168.1.0/24
The range would be 192.168.1.0-255 (with 0 is subnet id en 255 as broadcast id)
CCNA / CCNP / CCNA - Security / CCIP / Prince2 / Checkpoint CCSA
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Re: Subnetting Question 8 years 5 months ago #26651

  • Starfire
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www.subnettingquestions.com is a wicked little utility for honing your skills and with lots and lots of practice, making sure you figure out why your wrong each time, you'll have nasty little subnetting questions like this one in the bag and done mentally in under 15 seconds..... No Joke...

I haven't subnetted a thing for a year and a half until tonight and went on that web site (interview for networking job in a couple days) and was still getting over 90% correct in my head. (kept forgetting the broadcast address... doh!)
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Re: Subnetting Question 8 years 5 months ago #26656

  • SteveP
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As it's a class A address, the network address (before subnetting) is 77.0.0.0. The third octet is the "interesting" octet and the block size is 32 (256 - 224, or 2^5).

This means that the:

first subnet is 77.0.0.0 / 19
second subnet is 77.0.32.0 / 19
third subnet is 77.0.64.0 / 19
fourth subnet is 77.0.96.0 / 19

etc.

Note that the first and last subnets may or may not be usable, depending upon the router's configuration.
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Re: Subnetting Question 8 years 5 months ago #26665

  • S0lo
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Added to SteveP's. Here is the rule:

1. Keep the classfull octets as they are. In this case (Class A). Octet 1 will stay at 77

2. Zero out all bits of the subnet-work. In this case bit 1 in 2nd octet up to bit 3 in 3rd octet (counting from the left). This will be your FIRST subnet.

3. Increase the subnet-work bits by 1. This is your SECOND subnet.

4. Increase the subnet-work bits by 1. This is your THIRD subnet.

......

And so on until I die.... (just kidding :)). Host bits should be all zeros as SteveP did.

If the question asks for exact IP ranges. You would get the first and last HOST addresses for each subnet

First address: All zero for the host bits + 1
Last address: All ones for the host bits - 1

In such a case, Your final answer would be some thing like this:

first subnet is 77.0.0.1 to 77.0.31.254
second subnet is 77.0.32.1 to 77.0.63.254
third subnet is 77.0.64.1 to 77.0.95.254
fourth subnet is 77.0.96.0 to 77.0.127.254

Hope it's clear
Studying CCNP...

Ammar Muqaddas
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www.firewall.cx
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