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recommended network size

hi all

i currently support a network with a /22 bit subnetmask. 256 addresses are reserved for devices, such as server printers, network gear etc. and the rest for dhcp hosts, about 70% of ips i know of are used. The problem i've got is that over the years, the maintance of the ip allocations has gone somewhat astray and allocating new ips for devices is becoming increasingly hard. No set rules on ip allocation has been set and kept to. Recently i created a new /24 bit subnet to migrate new servers across to but was told that by doing this im creating a problem to fix a problem. To the best of my knowledge i cant see why this would cause any design issues. As this would address issues such as excessive broadcasts, reduced tcns for spanning tree, fixing ip allocation etc can anyone offer any advice? ir even point for and against the creation of the new network?

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Re: recommended network size

In the Cisco Press book "Top-Down Network Design" by P. Oppenheimer, it is recommended that a busy IP/multiprotocol subnet have a maximum of 200 nodes. So a /24 is ideal for a VLAN/broadcast domain.

If you search on that book title in the forums here, you'll see more detail on this (I've posted about it before).

Depending on your network's routing/multilayer switching capabilities and current traffic load, having everything with a /22 mask might be OK. It is possible that creating a separate /24 and then having to route between the two subnets might result in a bottleneck.

For example, if the /22 subnet has 500 10/100 Ethernet nodes, and you put servers onto a /24 that's a secondary IP address on a 10-meg Ethernet router LAN interface, then everything will jam up trying to go into and out of that LAN interface. Even if you have two LAN interfaces on the router, and they're 10/100, if the router's CPU is slow it may not forward at "wire-speed".

On the other hand, if you have the /22 and /24 subnets hosted in a non-blocking ("wire-speed") multilayer switch, then this really isn't much of an issue, if at all.

So, it all depends on how your network's set up.

New Member

Re: recommended network size

thanks for you reply.. i just wanted to confirm what i thought was correct.. the network is held together by 4 multi-homed cat5500, a cat6509 with a mfsc2 doing the routing as the core switch.. so im pretty confident that the router will be able to handle it without a problem.. this also answers my question of the selection of a /24 network size. to add to the issues the network still has legacy ipx traffic and also dec net stuff on there, LAT may stiff be around but no longer routered.. so as u can imagine there maybe a number of broadcasts floating around... just one other query.. does the selection of ip numbers really matter? as i choose the /24 to be 1 lower than that of the /22 network ie start of /22 is at x.x.45.x, i choose /24 to be x.x.44.x.. i was given the reason that if would be hard to expand the network..althought true. wouldn't it be better create a new network than expand an exisiting /22 to become a /23.. because of migration issues for hosts etc.. thoughts?

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