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On a consumer-grade router, if I set a subnet mask to limit the number of concurrent users (for example, 255.255.255.248), could this cause problems for those users (accessing the web, etc)? Do I have to be aware of anything upstream at the ISP, or does this only affect my LAN?

Clarification

This is the part I didn't get fully answered in my earlier question, in case it looks familiar. I did learn that:

  • I need to make sure that DHCP has an address range that fits within the subnet mask range.
  • I need to remember that a couple of IP addresses will automatically be spoken for by the access point and/or gateway (and maybe the broadcast address?)
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3 Answers 3

up vote 2 down vote accepted

The big part about subnet masks is that all the members of the subnet have to agree what the mask is, otherwise they may have trouble talking to each other.

So you could reduce your mask to 255.255.255.248, but all the computers you want to be able to use the router would have to have the same netmask. If you then want those computers to talk to other computers beyond that netmask, they will either need a second interface (physical or virtual) on the second network, OR a route to a router which knows how to get between the two subnets.

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"All the computers you want to be able to use the router would have to have the same netmask" Interesting. But they will be given the netmask by DHCP, right? –  Nathan Long Nov 23 '09 at 21:36
    
If that's the way you set it up, yes. The point being: the netmask that the computers have must be the same that the router has. –  David Mackintosh Nov 23 '09 at 21:50

You'll end up with only 6 ips communicating with the router, that the purpose of VLSM see RFC 950, but unless you have only a group of 6 computers that need access to outside your network you may may prefer using bandwith management.

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I assume you are using RFC1918 address (10/8,172.16/12,192.168/16) space and some kind of address translation. If this is true, what you use on your LAN for a netmask doesn't matter at all to the rest of the world.

If you are using publicly routable address space, then using an incorrect mask could cause lots of problems. It would be safe to use a smaller mask then what you have been allocated, but using a mask that is too large would cause you to have issues when you communicating with whoever owns the address space that have included in your mask.

If you are on a network that automatically updates routes of neighbor routers using something like OSPF, RIP, BGP, etc. you could cause issues on the entire network. But you mentioned this is a consumer router, so I doubt you are doing this.

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