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I've noticed when I reconnect to internet, the last two bytes of IP address static IP changes.

Does that mean when I disconnect someone else could have the same IP? How does this process differ from dynamic IP?

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This sounds very confused. – Ignacio Vazquez-Abrams May 18 '11 at 0:35
you're title is good and clear but your question itself is pretty vague, please clarify. – studiohack May 18 '11 at 0:39
doesn't sound that confused. The place where you see an error, Has a question mark after it. It's a simple question and a simple answer. First sentence ok. Second question Yes. Third question. It is dynamic ip so same process. – barlop May 18 '11 at 1:32

If your IP address is changing, you don't have a static IP address, but a dynamic one, and thus your answer is that "this process" does not differ at all from dynamic IP assignment because it is dynamic IP assignment.

And yes, it does mean that when you disconnect, someone else could get "your" IP address dynamically assigned to them when they connect.

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Ok, How do i check if i have dynamic IP? – user764841 May 18 '11 at 0:46
@user764841: If your IP changes, you have a dynamic IP. – Wuffers May 18 '11 at 0:49
Most (ALL?) ISPs will assign an ISP dynamically using DHCP. But they may give you the same IP address all the time (they call this static). The only way to tell if you have a static address is that it never seems to change. – Matt H May 18 '11 at 0:53
@Matt That's not strictly, entirely true. My ISP usually gives me the same IP, although it does change from time-to-time. They don't call it "static", though, and their static IP service (for an extra charge, of course!) is indeed static (although they offer it via static DHCP assignment if you don't statically assign it yourself). – Kromey May 18 '11 at 0:56
PPP (and PPPoE) doesn't use DHCP; the peer always assigns the IP address. – Ignacio Vazquez-Abrams May 18 '11 at 2:04

It is usually done the same way your home router does it, but with much more expensive equipment. It is more expensive because it is far more configurable. The ISP I work for uses an ERX to serve IPs, it stores mac addresses in its arp table and assigns ip address based on the card and port, which is mapped to a circuit through ATMs and eventually a DSLAM (or CMTS on similarly setup cable providers) - we statically assign, but not based on DHCP. This is the case with both bridged and routed circuits, where routed circuit simply have blocks on static routes over a /32 wan.

Other ISPs will use the arp table to assign addressing via DHCP, where static address are simply removed from the DHCP pool, but will appear on the lease table. This static is also usually mapped to a port and card on the router so you can only get it on your own circuit (but not always). In cases where you have a dynamic address, if your lease runs out and your device is not connected to renew, it will be handed out elsewhere and you will get the next available in the pool when you reconnect. Some leases last a few hours, some up to a week, depending on your ISP.

There are of course other configurations, but these cover the most commonly seen.

EDIT: I forgot to add, most single ips are /32 and are divided from a larger block. These blocks are usually assigned by region (or ISP in the case of smaller ones) which is why all IPs that you get will be very similar except for the last octet (or last two in the case of larger ISPs.

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You can reserve IPs in a DHCP (dynamic) setup. The same mac address will pull the same IP every time its turned on. This makes your internet connection faster as the router has 'memorized' where to send packets (your mac address/ip). This can be reset at the command line in the router, and everyone will get a new IP.

In a static config, they assign you an IP, and that is your IP. It will be your IP until you stop paying for service, in which they then terminate it, and assign the IP to someone else.

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