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We have a /24 IP Address range for PCs containing 240 allocated addresses at the moment.

Our company is planing for expansion of another 50 PCs, our existing IP range is 192.168.8.0/24

Do we add on another /24 (192.168.7.0/24) on the router? or Do we subnet 192.168.8.0/23 (255.255.254.0) on the router instead?

I would like to know which is the best option for our network.

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migrated from stackoverflow.com Jul 18 '12 at 10:06

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3 Answers

Since networking experience is lacking at your shop (or you wouldn't have to ask the question), your best option is to increase the subnet to a /23. That will give you a subnet with 256 additional addresses.

General rule of thumb is that you don't want more than a /24* on the same subnet, as you can run into problems with too many hosts on the same broadcast domain, so don't expand it to a /22. But there won't be any noticeable difference between ~240 hosts and ~290 hosts, so you should be fine. If you get to the point where you need the additional address space afforded by a /22, you really need to hire someone with networking experience to administer your network (at least as a part time role).

*:Academic (?) article with Cisco's maximum recommended number of hosts per subnet, based on protocols running in the broadcast domain. 500 for IP, 300 for Netware, 200 for AppleTalk, NetBIOS or Mixed. (NetBIOS is used by Windows, of course.)

*:Also, seems that the CCDA books now make their recommendation based on percentage of broadcast traffic, rather than host counts, and they state that subnets should be designed such that the amount of broadcast traffic does not exceed 20%. Might be a bit much for the OP to implement such testing.

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Do you have a reference for the 'rule of thumb'? It doesn't feel quite right to me. –  Iain Jul 18 '12 at 14:20
    
Yeah, it's from my CCNA material, which may be a bit dated for how long ago I took it, but that's still the Cisco position for the general case. I'll see if I can dig up an online statement to that effect from someone official... or officialish. –  HopelessN00b Jul 18 '12 at 14:24
    
Ah, found something, adding it to the answer. –  HopelessN00b Jul 18 '12 at 14:37
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By having a new subnet:

  1. the broadcast domain will be larger (I don't think it is a big deal in this size, 256 vs 512)
  2. By subnetting you have one (may be 2) additional IP (no IP for internal router)
  3. In your example, the IP of previous nodes need to be changed so may be 192.168.6.0/23 would be a better option than 192.168.8.0/23
  4. Also with subnetting you don't need a router! and additional hop for your traffic
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Thanks for your response. Another one thing i want to know, If I select 192.168.8.0/22 instead of 192.168.8.0/23. Is there there any possibility for network chocking and broadcast Or any degrade in network performance. –  user1533629 Jul 18 '12 at 6:34
    
Not for such a small amount of computers –  MichelZ Jul 18 '12 at 10:35
    
I would definately go for a subnet change, instead of adding another subnet, as subnetting like this makes things easier. –  Frederik Nielsen Jul 18 '12 at 10:52
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Instead of changing the netmask, you could switch another private ip-address range, like 172.16.x.y .. 172.31.x.y to have about 65000 hosts. Your users won't notice any change, just update the dhcp-server and optionally change your DNS.

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rfc1918 says that 192.168 is a /16 so you get ~64k hosts there. –  Iain Jul 18 '12 at 11:56
    
Yes, but if you use e.g. ifconfig eth0 192.168.7.1 up, it defaults to a netmask 255.255.255.0. To get 64k hosts, you have to add ` netmask 255.255.0.0` - that are 20 characters more. Keyboards could live much longer without this. –  ott-- Jul 18 '12 at 12:44
    
You must buy cheapo keyboards ;) –  Iain Jul 18 '12 at 14:18
    
I would suggest that creating a new IP address scheme and migrating 240 hosts (and networking gear) is a non-trivial operation, and based on the OP asking this question, his shop's probably better served by just expanding the netmask on the router (switch?), which is a fair bit easier for a shop without a network admin. –  HopelessN00b Jul 18 '12 at 14:27
    
Either way, the netmask needs to be changed on all the existing systems, not just the router. Otherwise, the existing systems will assume the new ones are in a different subnet and try to send it to the router, which won't route it, since it knows it's on the same interface it received it on. Unless you really want to start messing around with proxy ARP, which I don't recommend. –  mfinni Jul 18 '12 at 18:06
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