Dynamic IP question

[kumarkumar]

My ISP provided dynamic IP when I first got the connection (cable internet). They never mentioned whether it was dynamic or static. I came to know it by trying to renew the IP by executing "ipconfig flushdns/release/renew" command. At that time the IP would renew everytime I would pass the command.

But then as days passed by, IP renewal became difficult and more difficult. I would have to execute the commands a few times before the IP would renew. After the first couple of months I would have to execute the command twice approximately to renew, then after a couple of more months I would have to execute 5 times approx, then after a couple of months 10 times approx and so on.

Now it has come to a stage that the IP doesn't renew at all even after executing the command 100 times. It has become exactly like a static one.

My question is:-

Is this possible? I mean an IP "gradually" changing from dynamic to static. I was under the impression that an IP is either dynamic or static, nothing in between. If its static, IP would never renew by executing the above command, and if its dynamic, IP would renew everytime I executed the above commands. There is nothing in between.

Edited May 12, 2008 by kumarkumar

[tain]

My ISP provided dynamic IP when I first got the connection (cable internet). They never mentioned whether it was dynamic or static.

It is almost always Dynamic (DHCP). You usually have to pay extra for Static, if it is even an option. Sometimes it is not offered to consumers.

At that time the IP would renew everytime I would pass the command.

But then as days passed by, IP renewal became difficult and more difficult. I would have to execute the commands a few times before the IP would renew. After the first couple of months I would have to execute the command twice approximately to renew, then after a couple of more months I would have to execute 5 times approx, then after a couple of months 10 times approx and so on.

Why are you renewing your IP all the time? What are you trying to do? A Dynamic IP will use DHCP to renew the IP autmoatically. In both cases you don't have to do anything.

My question is:-

Is this possible? I mean an IP "gradually" changing from dynamic to static.

No, it is one or the other. But just because an IP is Dynamic doesn't mean that is has to or will change. Just that it might.

If its static, IP would never renew by executing the above command, and if its dynamic, IP would renew everytime I executed the above commands. There is nothing in between.

Those commands would not affect a Dynamic IP because the DHCP server hasn't issued an IP lease to your computer. Those commands will renew a DHCP IP, but renewing is not the same as changing. You are given the IP for a certain period of time (called a lease) and you can request to renew that lease to keep the IP. If the server decides that you can't have the IP anymore (there are many things the server will consider) then you may get a different IP. You have very little control over this.

Lots more info here.

[kumarkumar]

I actually meant "change" by the word "renew" in my statements in the first post.

But I guess they are not the same.

Who decides or how is the lease time decided? Is the lease time the same one that I see when I execute "ipconfig -all"?

If there is a lease time kind of thing, then could the IP be changed using the command before the lease time expires.

I also read that there is pool of IP addresses assigned to a client from DHCP and it may be exhausted. So what happens then? Does it cycle from the pool if IP addresses or it remains "stuck" at the final IP address?

[rendrag]

the lease time and circumstances are decided by the server admin of the DHCP server. The lease time is shown in the ipconfig /all.

Your IP will change only if the DHCP server wants it to. For example, if the admins were to set up static leases for some reason based on relative geographic area (unrealistic but possible), and they needed the IP you had currently leased, when your lease expired, your IP would change to whatever the admin assigned to your device and whoever they assigned to your old IP would get it when that device renewed.

In my home network, I can release and renew 1000x and get the same IP every single time. The DHCP server in my router sees no reason to change the same IP.

[Tripredacus]

I actually meant "change" by the word "renew" in my statements in the first post.

But I guess they are not the same.

Who decides or how is the lease time decided? Is the lease time the same one that I see when I execute "ipconfig -all"?

If there is a lease time kind of thing, then could the IP be changed using the command before the lease time expires.

I also read that there is pool of IP addresses assigned to a client from DHCP and it may be exhausted. So what happens then? Does it cycle from the pool if IP addresses or it remains "stuck" at the final IP address?

I can tell you how Adelphia's (now Time Warner) DHCP works (in certain areas) and hopefully it will answer your question.

The first time you boot your computer or connect it to the new internet (such as after it gets installed), the computer sends out a request via your cable modem for an IP address. This request is received by the ISP's DHCP server. The DHCP server records the MAC/Hardware address of your NIC, and then checks the active lease table to see if it has an IP address for you. Since this is the first time you ever connected this computer to the service, it doesn't find a record. The DHCP server then checks the pool of available IP addresses, selects one and marks it as "yours" and allows you to have it for a 3 hour period. No matter how many times you release or renew the IP address, you will always get this one.

In theory, after your 3 hours are up (the computer records its lease time as well and you can see this in ipconfig) your computer is supposed to send a renewal request to the DHCP server. Now this doesn't always work as planned because Windows remembers the IP address in the ARP table. So initially, your computer will ask the DHCP server to renew using the IP address it is currently using. If that IP address is either available or still assigned to that MAC address, it renews the reservation for that IP address for an additional 3 hours.

While you are actually receiving a dynamically assigned IP address, many people will note that their address doesn't change or doesn't change often. This behaviour depends on how many network connections (customers) are on the node/segment you are on in their network, and if you computer was on or off at the time your lease expired. Typically, the computer will be able to renew using the same IP address if the computer is on at the time of expiry. The only case where this doesn't happen is if there is an "offender" (a rogue or malfunctioning computer) on the network or the DHCP server runs out of IP addresses in its pool for the scope.

If your computer is off at the time of expiry, the DHCP server may reassign your old IP address to another computer, or perhaps it won't. Like I said before it depends on the amount of customers in that area. There are instances where the computer will use its IP address after the lease expired without permission of the DHCP server, but that's a whole other situation altogether.

If anyone has any other questions about how Cable internet works, I could probably answer them.