PC with static IP with a DHCP server in the LAN

[Zucarita9000]

I need a certain PC in the LAN to have it's own static internal IP (for remote access). I have a router in the LAN, wich has DHCP enabled. Do I have to set anything in the router of will it automatically "see" that one machine already has an IP and wont asign it's address to another machine? I also have to setup port forwarding to that specific IP.

 

[LBM1A]

Should be no problem there.. Just assign the IP address to the PC that you're wanting to have a Static or Hard Coded IP to. Got to the Properties of the NIC and change it from "Obtain IP Address automatically" to "Use the following IP Address" for Default Gatewary be sure to specify you Routers IP Address and also do the same for Preferred DNS Server.. All my PC are set this way..

LB

 

[kt]

To be safe, I would recommend you configure the DHCP on the router to assign IP addresses in a certain range just in case. That way you can make sure that none of the DHCP enabled computers will get an IP address that one of your static IP computers are using.

 

[Zucarita9000]

I see... configure the router to asign IPs from 192.168.1.2 to 192.168.1.30, and the asign 192.168.1.31 to the PC I want a static address. Is that waht you're saying?

 

[cleverhandle]

Yes, that's the idea. The DHCP server has no way of knowing whether a machine has grabbed an address from its range or not. So limit the range and take something outside of that.

edit: clarity

 

[LBM1A]

Dropping the Routers DHCP range down is not necessary, but if you want to go that route there should no problem. The Router holds the "in use" IP(s) in the ARP cache for a certain amout of days and as long as the PC stays sctive, then the IP will stay reserved.

 

[cleverhandle]

Originally posted by: LBM1A
The Router holds the "in use" IP(s) in the ARP cache for a certain amout of days and as long as the PC stays active, then the IP will stay reserved.

Interesting... I hadn't heard that before. Obviously, there needs to be an ARP cache for switching/routing to work. So the DHCP server is smart enough to read that? Do all consumer routers do this?

edit: can't type today

 

[LBM1A]

The way it works or so I've been told is: When an incoming packet destined for a host machine on a particular local area network arrives at a gateway, the gateway asks the ARP program to find a physical host or MAC address that matches the IP address. The ARP program looks in the ARP cache and, if it finds the address, provides it so that the packet can be converted to the right packet length and format and sent to the machine. If no entry is found for the IP address, ARP broadcasts a request packet in a special format to all the machines on the LAN to see if one machine knows that it has that IP address associated with it. A machine that recognizes the IP address as its own returns a reply so indicating. ARP updates the ARP cache for future reference and then sends the packet to the MAC address that replied.

I can only assume most off the shelf products do it this way.

LB

 

[cleverhandle]

You're describing ARP. I'm familiar with that. But just because the switch component has an entry in the ARP cache doesn't mean that the DHCP program is aware of that. DHCP and ARP work at very different levels.

 

[LBM1A]

Correct... but the DHCP server should also record the lease and hold the same information..

 

[skyking]

Originally posted by: LBM1A
Dropping the Routers DHCP range down is not necessary, but if you want to go that route there should no problem. The Router holds the "in use" IP(s) in the ARP cache for a certain amout of days and as long as the PC stays sctive, then the IP will stay reserved.

I recently had a router give out an active IP, one belonging to one of my servers, so i would use the range restriction.... It is not difficult to do, and makes certain that there is no conflict. I was building someone a new freebsd server, and it "stole" the IP of one of my current servers, LOL!
The console messages from my server were interesting, it was not too keen on that identity theft

 

[cleverhandle]

Sure, if the machine gets an address via DHCP. But the situation was to set up a machine with a static configuration on the DHCP segment. If you do that the machine will never talk to the DHCP server, so leases are irrelevant.

I'm not saying that your description is impossible, just that there's nothing in the protocols that demands it. It would have to be some kind of add-on by the manufacturer. Unless you can provide docs to that effect, I'll remain skeptical.

 

[Bleep]

Well here is what I do. I set the router to assign DHCP starting at .100 and then I have my server set to static,.20 Linksys says that you cannot do this (a mixed system) but I and probably many others have. Works great.

Bleep

 

[fargus]

Something else to try... don't know if your router has it, but my SMC has an option under DHCP called "fixed mapping"... basically you just tell the server to always assign a particular IP to a particular MAC address. Simple, and it works great, as you don't have to play with the settings on the PC at all. Some servers also have an option to exclude a certain address or addresses from a range.

 

[Zucarita9000]

Originally posted by: fargus
Something else to try... don't know if your router has it, but my SMC has an option under DHCP called "fixed mapping"... basically you just tell the server to always assign a particular IP to a particular MAC address. Simple, and it works great, as you don't have to play with the settings on the PC at all. Some servers also have an option to exclude a certain address or addresses from a range.

I'll check it out. The router is a Linksys BEFSR41

 

[Fuzznuts]

Originally posted by: Zucarita9000

Originally posted by: fargus
Something else to try... don't know if your router has it, but my SMC has an option under DHCP called "fixed mapping"... basically you just tell the server to always assign a particular IP to a particular MAC address. Simple, and it works great, as you don't have to play with the settings on the PC at all. Some servers also have an option to exclude a certain address or addresses from a range.

I'll check it out. The router is a Linksys BEFSR41

A static DHCP ip lease is generally called a reservation (no indian / native american jokes please ) have a look in your manual for that term if you get stuck.