DHCP Scope is running out

[shirzay]

Our DHCP Scope for our curriculum network is bout to run out, and soon there wont be any IP addresses available for the new clients (as we are going to buy another 50 PCs for our new ICT suite).

The DHCP scope is configure as:

IP 10.8.100.0 to 10.8.100.254

Subnet mask 255.255.255.0

Since 24 bits are assigned to the network part it only gives us 254 hosts.

I need some guidance in how to go about changing the scope to (16 bits for network part of the IP rather than 24), and what are the factors that i need to consider (such as changing router and switches configurations, software that uses IPs of the servers, web applications, etc, etc) before changing anything as it is a live network, and i can not afford for anything to go wrong.

 

thanks

[elsiegee40]

You appear to be using a class A network id prefix (10) with a class C subnet mask (255.255.255.0)

 

You really need to be using class B (which uses a 255.255.0.0 subnet mask) - these should start with a number in the range 128-191

 

I'm not an IP expert however, hopefully someone will be along soon to let you know the effects of tweaking your subnet mask to get a bigger range of addresses

[SYNACK]

You may need to be careful if you are part of a larger network and your address range is assigned by them. If you are expanding your network range without talking to them could cause an overlap.

 

Currently you are using a class A private addres space which means that you could configure it to have a subnet mask of 255.0.0.0 if you wanted to.

 

If you change the address mask in the DHCP settings to 255.255.0.0 you should be able to expand this address range and keep all of your exisiting static IP addresses the same. You will however need to make sure that you change the subnet mask on each staticly assigned ip address ie servers, printers because otherwise they will not be able to communicate with any equipment that is not within the 254 addresses that you were using before.

 

If you have a network that is split up into subnets internally then this will be more complicated as you will need to change routing tables as well.

 

The avalible private subnets (for internal network use) are:

Class A: 10.x.x.x mask 255.0.0.0

Class B: 172.16.x.x mask 255.255.0.0

Class C: 192.168.x.x mask 255.255.255.0

 

You can divide these up into smaller subnets however you wish by increasing the value of the subnet mask.

Edited January 25, 2008 by SYNACK
more information

[elsiegee40]

Thanks SYNACK for confirming what I thought!

[sahmeepee]

The DHCP scope is configure as:

IP 10.8.100.0 to 10.8.100.254

Subnet mask 255.255.255.0

Since 24 bits are assigned to the network part it only gives us 254 hosts.

I need some guidance in how to go about changing the scope to (16 bits for network part of the IP rather than 24)

 

As hinted above, you don't necessarily need to do this by whole octets. That is, you can have a subnet mask of (say) 255.255.252.0 which will give you approx 4 times your current address space:

 

11111111.11111111.11111111.00000000 = 255.255.255.0 - current setting

11111111.11111111.11111110.00000000 = 255.255.254.0 - ~2x as many hosts

11111111.11111111.11111100.00000000 = 255.255.252.0 - ~4x as many hosts

11111111.11111111.11111000.00000000 = 255.255.248.0 - ~8x as many hosts

 

I strongly suspect your IP range is part of a larger network though - 10.8.100.x would be a very random subnet to pick for a "standalone" network. Definitely check with the people who maintain the larger network as SYNACK says.

[shirzay]

As hinted above, you don't necessarily need to do this by whole octets. That is, you can have a subnet mask of (say) 255.255.252.0 which will give you approx 4 times your current address space:

 

11111111.11111111.11111111.00000000 = 255.255.255.0 - current setting

11111111.11111111.11111110.00000000 = 255.255.254.0 - ~2x as many hosts

11111111.11111111.11111100.00000000 = 255.255.252.0 - ~4x as many hosts

11111111.11111111.11111000.00000000 = 255.255.248.0 - ~8x as many hosts

 

I strongly suspect your IP range is part of a larger network though - 10.8.100.x would be a very random subnet to pick for a "standalone" network. Definitely check with the people who maintain the larger network as SYNACK says.

 

sorry for my ignorant but I don’t have an idea about who would be the people who maintain the larger network? Would it be Local Authority?

[Heebeejeebee]

Possibly whoever supplies you internet provision. IE RM issue IP address ranges for the SWGfL. We have to contact them if we want to make any radical changes to any our our subnets or to change the range.

 

HBJB

[russdev]

Larger network could be LA lets take embc for example we are given an range out of 10. As this means that lea internally can see all our local machines and we also have selected few ips that can be seen by other schools if wish to do school to school stuff.

 

Russ

[z4ydi]

I will find out whether it is the LEA, but please let me know whether it will be an easy process to change the scope

[russdev]

Contact rbc helpdesk again if in embc network like I am don't just change it as it will start causing issues.

 

Russ

[laserblazer]

I still cannot get my head around this We have an admin and curriculum server both sharing the same network. When I arrived the IP address range was 1.0.128.x The curriculum server was on 100 and the admin server on 200 with admin machines having static IP addresses up to 210. The curriculum machines had a DHCP scope from 110-190 or thereabouts. The reason for this IP address was because we had Acorn machines with fixed IP addresses. The SubNet was 255.255.255.0

 

I replaced our NT4 curriculum server with a new 2003 server and we were the first school in the County to do so. It was over Xmas and I had the devil of a job. Viglen tech support were no help. In the end re-installed server 2003 and got it to work with the exception of DHCP. By this time I was pretty p*ssed off with it and my Xmas dinner was on the table, so I assigned static IP addresses to every computer.

 

When we moved to the County broadband we were given a new IP range 10.91.4 x.

 

When we replaced the Admin server in November, I got the LEA to come in and do it (wise decision because it went pear-shaped) and took them ages. So when I was constantly bombarded with "When will we be able to get our computers back?" I was able to shrug my shoulders. Anyway, they said the subnet mask should have been 255.255.252.0 which is what we have set the admin machines to.

 

I now have machines and printers with IP addresses anywhere between 10.91.4.1 (gateway) and .255 and the curriculum machines are still on 255.255.255.0

 

If I change that to 255.255.252.0 what additional IP addresses will this give me. I want to get DHCP working on the curriculum server but I don't have asufficient contiguous range of addresses available without changing the admin server addresses, which I would rather avoid.

 

Sorry this is so long but I thought the history would help and you can see how thick I am!

[SYNACK]

Your adress space with a subent mask of 255.255.252.0 is between 10.91.4.1 and 10.91.7.254 this gives you 1022 avalible addresses all in the same subnet. The ip broadcast address is 10.91.7.255 and the network address is 10.91.4.0

 

hope this helps

[SYNACK]

Sorry about the dual post but there is no edit in pda mode.

 

If you change all of the curriculum machines to the same subnet mask this will extend the range of ip addresses that they are allowed to send and recive traffic from without involving a router. This will allow all of the computers on the network to talk to any other computers located in your subnet rather than restricting them to the 255 addresses in use previously.

[powdarrmonkey]

 

Is it a simple process of doing the following:

 

 

Yes, basically, though you don't need to stop and start the service while you're doing it. But you must ensure if you're part of a wider network (ie. check with your ISP) that you use a range they allocate you, not just make it up. Otherwise you will have all sorts of problems, and so will everyone else on the wider network.

 

You then need to modify the static hosts appropriately.

 

Edit: You also don't particularly need to change your addressing scheme (unless you use subnetting, which is unlikely). Subnet classes don't really have any meaning on today's internet. If you're interested, google for Classless Inter-Domain Routing, or CIDR.

[dezt]

I had the same problem not too long ago, i resubnetted from 255.255.255.0 to 255.255.254.0, here is the link to the Microsoft article that I used to help me understand it.

 

Increasing the number of IP addresses on a subnet in DHCP Server

[powdarrmonkey]

Yes, you need to delete the scope. Make sure you've written down your reservations and scope options first

[dezt]

Yes, you need to delete the scope. Make sure you've written down your reservations and scope options first

 

That's exactly what I did.

[powdarrmonkey]

Do you have anything set up like VLANs, routers, etc?

[powdarrmonkey]

Did you not get your new scope from them then? If that's the case then yes, you should wait to hear from them.

 

But you must ensure if you're part of a wider network (ie. check with your ISP) that you use a range they allocate you, not just make it up. Otherwise you will have all sorts of problems, and so will everyone else on the wider network.

 

As you've just demonstrated.

Edited August 5, 2008 by powdarrmonkey

[Face-Man]

What's your default gateway for the 3.x addresses. The fact you can't ping local addresses is a bit worrying but it maybe that the client has not renewed it address from your new DHCP scope and still has an old 255.255.255.0. I would also check if your address scheme is allocated by the LA you really don't want to be using another schools addresses (thats assume you dont have NAT or a firewall infront of your internet connection)

[ahuxham]

Make sure the default gateway is setup correctly.

 

For example our Vigor 3300V router/firewall will only accept connections on 192.168.0.x/22

 

However we have 192.168.1.x/22, 192.168.2.x/22 and 192.168.3.x/22 that cannot access the router.

 

(/22 = 255.255.252.0)

 

DHCP > SCOPE > SCOPE OPTIONS > 003 (Router Name) set to the Router IP.

[ahuxham]

Originally Posted by powdarrmonkey

But you must ensure if you're part of a wider network (ie. check with your ISP) that you use a range they allocate you, not just make it up. Otherwise you will have all sorts of problems, and so will everyone else on the wider network

 

If your router supports NAT, you can do anything you want, no need for authorization.

[powdarrmonkey]

@ahuxham: the default gateway doesn't matter in this case, packets within the same subnet are sent by broadcast. If the connection is from an LA, as it sounds, then they almost certainly provide the edge router and gateway so the OP won't be able to do anything with it anyway.