Need help getting to grips with subnetting

I am trying to get my head around sub-netting but not doing very well. Every time i look at examples in books and online i get in a mix. I can only really understand it better when it is relating to ip address ranges i am used to working with.

I am mainly keen to find out about what to do when you use up all the ips in your range. So if you have for example 1000 ips to dish out what happens when they are all used up and more clients need address?

Would some one be so kind and offer me help please to get my head round this bloody thing.

I've found these very helpful - Firewall.cx and LearnToSubnet.com.

Presumably your doing your CCNA exams, thus you just need to work out subnet masks from bit masks?

Do you want to give us some numbers to talk about e.g. current ip range and network mask?

regards

Simon

Having recently run out of IP addresses in the subnet allocated to me by the LEA I know exactly what you have to do.

You get a second subnet or a bigger subnet.

A subnet makes a lot more sense when you convert the subnet mask to a single binary string - then you will find it will be a list of 1's then 0's. The number of 1's can vary (and therefore obvisouly the number of 0's) but they are never mixed so:-

11111111111111111111111100000000

or

11111111111111111111111111100000

But never

11111111111010011100111001010010

So my current subnet mask here is 255.255.252.0 which converted to binary is

11111111 11111111 11111100 00000000

So my first 22 digits are 1's and the remaining 10 are 0's

If you then converta any IP address in my subnet then you would see that the first 22 digits are identical to each other and the only digits that ever differ are the last 10. This is because the first 22 digits identify the subnet that the IP address lives in as determined by the subnet mask.

As you only have 10 binary digits with which to create your unique ip addresses for you PC's then you are limited to 1024 (in reality it would not be 1024 as some IP addresses have specific roles in the subnet - but do not worry about that at this stage)

In order to have more IP addresses you need to use more than 10 binary digits - so using 11 binary digits would give you 2048 unique IP addresses which would mean a different subnet mask where the last 11 digits were 0's and therefore only the first 21 digits can be used to identify the subnet which in turn means there are less subnets available.

A subnet allowing 2048 IP address would be

11111111 11111111 11111000 000000

or

255.255.248.0

At least - I think that is the way it works!!

I didnt think the LEA could extend the range that is given to a school, only give another range. If they could extend the range, surely that means 1 of 2 things. The LEA has a larger range for each school but only assigns part of it or reassign everyone.

Indeed, it would depend how they do their capacity planning (or lack of it).

However having two subnets compared to just one has a performance implication. Your router(s) must route traffic between the subnets. Although I believe you can avoid this by pushing out a static route setting with DHCP as an option in your leases.

Another alternative would be to use a NAT firewall at your border and use a private IP range on your network.

Quote:

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Originally Posted by Kyle

I am trying to get my head around sub-netting but not doing very well. Every time i look at examples in books and online i get in a mix. I can only really understand it better when it is relating to ip address ranges i am used to working with.

I am mainly keen to find out about what to do when you use up all the ips in your range. So if you have for example 1000 ips to dish out what happens when they are all used up and more clients need address?

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3 Com publish a good document on understanding IPv4 addressing, it covers CIDR and VLSM these are areas you need to look at you will find it here

limbo-- great answer!, many thanks, should come in useful as i wans't really sure of it either.

Quote:

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Originally Posted by limbo

Having recently run out of IP addresses in the subnet allocated to me by the LEA I know exactly what you have to do.

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Just out of curiosity how many IP addresses did your LEA assign you? We have been given loads more than we will ever need.

Tom

1022 here. We deal with the issue of running out of ip addresses by setting up appropriate lease times and for the most part that works. If our network increases anymore then we would have to look at what Geoff has mentioned and create our own private network (easy enough to setup).

The CIDR article in Wikipedia is a good starting point as well.

http://en.wikipedia.org/wiki/Classle...Domain_Routing

Quote:

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Originally Posted by Geoff

Another alternative would be to use a NAT firewall at your border and use a private IP range on your network.

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This is by far the best option and it also adds an additonal layer of security but you must remeber that acces to any device on your LAN behind the firewall will require "double natting"

Quote:

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Originally Posted by djm968

This is by far the best option and it also adds an additional layer of security but you must remember that access to any device on your LAN behind the firewall will require "double natting"

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Personally I would use a DMZ . I have talked about this kind of setup previously.

http://www.edugeek.net/index.php?nam...&p=69060#69060

The diagram in the post illustrates my preferred setup for a DMZ (even though in that context we're not discussing them).

Sorry - been away short listing for the last couple of hours so just catching up.

Our subnet is 255.255.252.0 so we have around 1000 IP addresses. The LEA have a few for routers, proxy servers VC equipment etc. and then we have a lot of laptops (couple of hundred) that switch between being docked with a hardwire and then roaming on wireless which double the number of IP addresses the laptop uses.

The problem was most noticeable when we were setting up new equipment in parallel to the equipment it was going to replace - hence there being an extra couple of hundred machines on the network. We also have a few wireless PDA's just to add to the volume.

The lease is cut right down to a couple of hours but still get the odd problem until we can do the swap.

The LEA are providing us with a new, larger subnet. We toyed with the idea of a seperate internal subnet with a router or likewise acting as a gateway between the two - but in the long run it is not going to be a huge job to transfer over to a new range as most of our kit is DHCP anyway - just the servers and printers to swap over.