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Networks DHCP playing tricks on me?

Discussion in 'Tech Support' started by Tomm, 19 Jul 2009.

  1. Tomm

    Tomm I also ride trials :¬)

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    I have a problem with my DHCP - for some reason my computer and my router seem to disagree about what my IP address is.

    I've just changed routers, and although the new router seems to be working fine (I can access the internet and network shares via a workgroup), I want to set a static IP address to allow me to forward some ports. This is something that I've done in the past with no problems (with a different router).

    Have a look at this picture: (sorry about the size)

    [​IMG]

    On the left is my router config page, showing the DHCP settings. NB all the IP addresses are in the 192.168.1.x range where x<10. My PC is shown as 192.168.1.5, and the computer name is blank (it should not be blank).

    On the right you can see that the details for my LAN connection suggest an IP address of 192.168.1.81, as I've set. I have also shown the routing table although that doesn't mean a whole lot to me...

    The exact same thing is happening with my NAS, which I have also tried to assign a static IP to. The NAS thinks it is 192.168.1.78 (And this is the address I can use to SSH into it) but the router says 192.168.1.3 (and omits the name).

    I've tried restarting everything several times, and trying different static IPs, nothing seems to have worked. Any ideas?
     
  2. Gareth Halfacree

    Gareth Halfacree WIIGII! Staff Administrator Super Moderator Moderator

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    The machine in the DHCP table with IP 192.168.1.5 is not your PC. From what you've posted, your PC has a MAC address of 00-06-4F-77-B2-C9. The IP in DHCP is assigned to 00-22-69-03-B6-FC, which is a completely different manufacturer - and hence machine.

    If both systems are working, don't worry about them not appearing in the DHCP table on the router: they will either not show up, or show up with old details. When you set a static IP, the system never makes a request to the DHCP server - and so the DHCP server has absolutely no idea what machine name or MAC address is assigned.

    If you really want your machines showing in the DHCP table, you'll need to reserve the address on your router. By setting a static entry in DHCP for a particular MAC address, you can keep the system configured for dynamic configuration while still giving it a static IP each time. That'll also mean that the system is listed in the DHCP table.
     
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  3. Tomm

    Tomm I also ride trials :¬)

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    Oh OK, that sounds like a lot of sense. Thanks! So if I want to set some port-forwarding rules, I can just use the IP address that my computer has assigned to itself?
     
  4. Gareth Halfacree

    Gareth Halfacree WIIGII! Staff Administrator Super Moderator Moderator

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    Yup. The only thing to watch out for is when you've manually assigned something on a machine which is within the scope of the DHCP server. In the unlikely event that you have 80-odd machines connected to your router, the DHCP server will happily assign 192.168.1.81 to another machine - leading to an IP conflict.

    You're best off configuring the scope to only assign addresses from 192.168.1.1-128 (or similar). That way you've got 192.168.1.129-254 to set as manual assignations.
     
  5. Tomm

    Tomm I also ride trials :¬)

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    Now that you mention it, what is the most connections you could have with a bog-standard home router? In theory you can do 253 I think, but I always imagine they'd struggle horrendously with more than about 6 or 7?
     
  6. Gareth Halfacree

    Gareth Halfacree WIIGII! Staff Administrator Super Moderator Moderator

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    You shouldn't have a problem with a couple of hundred machines connected to a home router - the memory assigned to the ARP table is usually at least enough for a /24 in even the cheapest of devices. The only issue you'll get is blocking bandwidth and limits on the number of open connections each will be allowed to have to the WAN.

    I used to run a /24 on a Draytek Vigor, which isn't exactly enterprise grade, with no issues.
     

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