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.
45 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.
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.
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.
Any DHCP server, including those of your ISP, can give a client any IP it wants. It can give a client the same IP over and over. It can be set to give a specific MAC address the same IP each time.
You are not guaranteed to get a different IP each time you ask for one. Indeed, if this were the case, a malicious client could exhaust an IP pool very easily and deny service to other clients.
DHCP works on a lease model - your DHCP client asks a server for an IP, and the server leases the IP to the client for a specified amount of time. If that time expires and your client hasn't contacted the server, the IP is up for grabs, though this still isn't a guarantee you'll get a different one. If you are still within lease, it's common for the server to simply provide the same IP you were using before - this covers situations where your system reboots and asks again for an IP.
If you wish to avoid MAC Spoofing, you could buy a 2nd router from your ISP and a physical switch box. Then if you had the need to change your IP (or rather the MAC address associated with your IP), you would just switch to your secondary router with it's own unique MAC address.
As I understand it, each physical router from an ISP has a unique MAC address associated with it. You can setup an input (a or b) switch-box to control which of the routers the incoming line points to. Flipping the switch will give you a "new" MAC address, after which you just need to replace the first router to make the whole setup ready to use again.
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