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I have run ipconfig /all, which tells me DHCP is enabled, meaning I should have a dynamic IP address. However, no matter how many times I restart the modem, I end up with the same IP address. What am I missing?

I am trying to configure the reconnect option for a program called jdownloader, but before that, first I have to be able to reconnect and change my IP manually, which I can't seem to do. Clicking on ip v4 option and going to properties shows that "Obtain IP adress automatically" is selected.

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Why do you want a new IP address? Who are you hiding from?? –  Bigbio2002 Apr 4 '13 at 18:16
    
can you right click on your lan icon >> click on TCP\ip v4 option >> click on properties and tell me wether obtain ip dynamically is selected or static IP is selected.?? –  adithya Apr 4 '13 at 18:19
    
I am trying to configure the reconnect option for a program called jdownloader, but before that, first I have to be able to reconnect and change my IP manually, which I can't seem to do. Clicking on ip v4 option and going to properties shows that "Obtain IP adress automatically" is selected. –  Bankers Union Apr 4 '13 at 18:23
    
I use Jdownloader too, used to be able to aquire a new, but not any more. Mabey if i go back to dial-up :-) –  Psycogeek Apr 4 '13 at 18:33
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3 Answers

You're missing the fact that many DHCP servers, without further configuration, will give each device, as identified through its MAC address, the same IP address – even when they ask for a new lease.

Of course you can configure DHCP servers differently, but it's not uncommon for these leases to be reserved for a certain period of time, say a week, or a month. This is called reservation, and it may be enabled by default. It's just really useful: Imagine you take your laptop to work every day and then come back home. You will get the same IP address and won't have to look it up every time.

If you're talking about your router and your LAN IPs, look into its settings whether there's a table for IP address allocation that's automatically filled, and clear it. If you're talking about your external IP given to you by your ISP, there's not much you can change here, really. The ISP uses the MAC address of your modem (which they handed to you) to know it's you who connects. Changing the MAC address won't help you get a new IP—you just won't be able to connect to the Internet.

As @tictacbum correctly mentions, ipconfig will only show your LAN IP address, not the WAN address (the external IP). To check this, use any online service for that, or just Google.

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In my situation, it would really be more useful if I can reset my IP when I wish. How can I configure DHCP to disable reservation? I have a netgear WNR2000V1. I've checked it's settings, and it says Its configured to obtain IPs dynamically. However, since the IP isn't changing, I'm assuming it's because of the reservation system you mentioned. –  Bankers Union Apr 4 '13 at 18:28
    
Is it the external IP given to you by your ISP you want to allocate? Or just for your computer, e.g. 192.168.1.123? –  slhck Apr 4 '13 at 18:29
    
@BankersUnion, You cannot change the DHCP server at all. There is nothing you can do with DHCP to change what you are givien. As slhck mentioned, DHCP associates the reserved IP with a particular MAC. Many routers router permit you to set a different MAC address. Simply update your MAC, and viola, new IP. Don't do that too much though, or your ISP will get annoyed at you. –  Zoredache Apr 4 '13 at 18:32
    
I would like to change the external IP given to me by my ISP. –  Bankers Union Apr 4 '13 at 18:32
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@BankersUnion Well, if your modem/router allows you to change (spoof) the MAC address, and this won't let you connect to the Internet, that's not surprising, because your ISP knows your MAC address (because they gave you that modem) and uses that to give you Internet access. I'm afraid you're out of options. –  slhck Apr 4 '13 at 18:41
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The only way for you to have any control over your external gateway IP address is to pay your ISP for a range of static addresses. Then you are free to use any address within that range, as long as you don't use the same address on two different devices.

Otherwise, you are stuck with whatever address their DHCP server assigns to you. And, according to the rules of the DHCP server, you usually get the first available address, and you keep it until you disconnect completely, for a couple hours, until the address that was leased to you is reissued to someone else. Then you would get a different address when you switch your modem back on.

And don't try to put your own DHCP server on a DMZ or the WAN side of the network. Your ISP will call you very quickly and give you a couple hours and tell you to shut it down before they disconnect your service.

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There can be following reasons why DHCP server of your router is assigning same IP everytime you connects your device:

  • MAC-IP Binding. It can be the case if someone other than you has configured DHCP server. If this is the case, remove the binding.

  • Long IP Lease duration. DHCP servers assign and reserve IPs to devices for certain duration. If this lease duration is long (check router settings or manual), release those client leases. If your router doesn't support it, you can try "MAC Spoofing".

  • Low no. of IP assignments. Most of DHCP servers assign first available IP from the pool. So, your device can get the same IP even after lease is cleared because its the first available IP. In this case, only "MAC Spoofing" can rescue you while that annoying IP is reserved for a fake MAC.

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