Re: IP subnetting

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Beginner wrote:

I have a 126 IP addresses on a single subnet all routing through the same gateway. I have upgraded my router so I now have 2 interfaces. I want to put an SMTP and HTTP server on the 2nd interface and keep the internal hosts on the original interface. I think this is basically a DMZ configuration.

My current IP address assignment is sporadic, with some static hosts at one end or the other on the IP block and DHCP given a pool from the middle. I want to assign a /29 block of address from within my range to the 2nd interface giving me 5 addresses to use. I am a little unsure what the impact of this change will be on other network services, in particular DHCP. Will I be turning my simple single subnet into 3 different subnets? Do I configure my dhcp.conf with 3 subnet declarations? Can 2 subnets share a gateway address even if it's not local? Would it be advisable to re-configure those static hosts at one end of the block into the other end so I only have 2 subnets? What other services might be effected by this change? I can think of a few httpd.conf allow statements that might need changing and possibly some smb.conf changes.

Over 10 years ago, I had an office set up with a class C network. We installed a terminal server with 16 dial-up lines, each with a modem for dial-in. I reserved 32 IP addresses for office users to dial in on, and their IP addresses were set based on a dial-back scheme used by the terminal server. The IP addresses were all taken out of our class C network as a lump at the end. I viewed it as a subnet of our network, but in reality, I didn't have to. The TS was capable of proxy-arp for the remote users, so they looked to the office network like a part of the office network, even though they were all routed through the TS. It made life simple. It would probably have been much more complicated if we didn't use that particular TS or at least one capable of doing proxy-arp. I'm sure you could set up something similar, so long as whatever you are using to separate the second network is capable of proxy-arp for the second network and will do the routing for you.

A.B.C.0/24 <-----> TS <-------> A.B.C.224/27  dial-in machines

Kevin J. Cummings
Registered Linux User #1232 (
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