How often does a ISP change an IP if they use DHCP?

Discussion in 'Networking' started by Nocturnal, Apr 5, 2006.

  1. Nocturnal

    Nocturnal Lifer

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    I swear I had my last IP for over six months. I never turn my computer off so I know it has something to do with that. How long does a computer have to be off before the DHCP server will re-assign that IP to someone else? If I keep my computer on indefinately will my IP probably stay the same (so long I don't change NICs)?
     
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  3. Kelemvor

    Kelemvor Lifer

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    All depend son how they are set up. But even when it expires, chances are when it renews it will jsut grab the same one back again anyway unless something else grabs it before it can renew.
     
  4. Rapidskies

    Rapidskies Golden Member

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    Some ISP's give you back the same IP over and over. I've had other ISP's that the IP changes all the time.
     
  5. JackMDS

    JackMDS Super Moderator<BR>Elite Member
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    Just like there is No uniform schedule to changing Socks,:shocked: every thing is possible.

    The most prevalent scheme are.

    PPPOE DSL every time you log On.

    Cable, few months regardless of log ons.

    :sun:
     
  6. MixMasterTang

    MixMasterTang Platinum Member

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    Ipconfig /All from a command line should tell you what the lease expiration is. Different routers/firewalls may also tell you if you look at the configuration.

    Lease Obtained. . . . . . . . . . : Wednesday, April 05, 2006 9:34:00 AM

    Lease Expires . . . . . . . . . . : Wednesday, April 05, 2006 9:34:00 PM

     
  7. Madwand1

    Madwand1 Diamond Member

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    It seemed to me that my IP switched much more frequently after I registered with a (free) DDNS service. Could this have been a plot to get me kicked off the DDNS service for switching too frequently? Could it be a Heisenberg sort of thing, where it changes if you observe it? :)
     
  8. randal

    randal Golden Member

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    Admittedly, PPPoE generally changes with every logon/logoff, but a lot of places support the renegotiation of a client-requested IP address. I know that we do, so even though we are very dynamic, if a client asks for a particular IP and it is open in the pool, they can have it. Most places, though, oversubscribe or come very close to it, their IP pools which results in people never being able to get the same IP ... so they turn off the functionality. I guess we just have IPs to burn :)

    That exact scenario occurs in standard DHCP configs in ethernet-based environments (DSL is not ethernet, it is usually ATM, while Cable providers still use Ethernet at L2) - every time the client connects, it will ask for the same IP address it had last time. The DHCP server will then check its tables and ping the requested IP to see if it is in use. If it is not, then it will (almost always) re-issue the client the requested IP address. This makes most DHCP networks relatively stable and reduces the overhead in updating namespaces etc.

     
  9. JackMDS

    JackMDS Super Moderator<BR>Elite Member
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    Nice informative post randal.

    I have one installation in NYC with RR Cable. The Cable's Modem is unplugged every week for three days in a row.

    Yet when it is plugged back it would come with the same IP for 5 weeks.

    It is a little hard to believe that the IP would Not be snatched in three days time week after week.

    This let me thinking that for 5 weeks there in a policy of not releasing the IP as long as the MAC checks the same regardless of the status of the LogOn.

    :sun:
     
  10. randal

    randal Golden Member

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    Why would the IP be snatched? DHCP pools go from #0 to the end of the pool and -then- recycle. So if the pool is 256 IPs and you are customer number 76, you get (say) .77. As they add new customers, they grow up to 240. If somebody goes offline/online, they will request their existing IP to make the pool contiguous. If a -new- customer comes up and does not request a particular IP, they get the next available IP -- IP #241. So it goes until the pool is full. Once the pool has been allocated in its entirety, it will attempt to use not-currently-in-use IP addresses; this is oversubscription at the ISP level and is a really bad thing. Normally, they would expand the pool by a good 15-20% ... resulting in all of the existing customers getting the same IP over and over and over.

    Generally, the only time that a device will get a different IP on an ethernet/dhcp network is if, for some reason, it loses track of what IP address it had the last time it was on. Then it will request the next available IP. Also, the ISP can force the DHCP server to not honor requests for particular IPs, which results in everybody getting new IPs the next time they reconnect. After the first connection, "dynamic" host control protocol usually isn't :)
     
  11. Madwand1

    Madwand1 Diamond Member

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    Why would anyone want to rely on the ISP's whims and circumstances when working DDNS can be used for free?
     
  12. randal

    randal Golden Member

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    The ISP generally using dynamic DNS for their internal forward and reverse resolution. This is also built into DHCP and is not the same thing as dynDNS.com, zoneedit.com etc.

    An example usage of this would be on a corporate network where a laptop named "randalslaptop.corp.company.com" moves around the company network quite a bit. That laptop gets plugged in at an executives office to troubleshoot, at my desk, at a random workstation, etc. Everytime I connect to the network, I'm probably on a different DHCP and most assuredly on a different subnet, meaning that my IP will change several times. Every time my laptop gets an IP with DHCP, it will register itself with the DHCP-Provided DNS servers. The DNS servers will then update their zones so that "randalslaptop.corp.company.com" resolves to whatever IP address I am currently using. Additionally, the reverse DNS PTR entry for that IP address, say 192.168.72.28, will then be updated to resolve to "randalslaptop.corp.company.com" ... With this feature enabled, it is extremely easy to locate a computer anywhere on the network and also very easy to maintain any connections or dependencies that may be required.

    An example of -that- would be me leaving a laptop at an office somewhere else in the building, going back to my desktop and then using remote desktop to connect to `randalslaptop.corp.company.com` (which will always point to my laptop) instead of having to lookup and memorize my IP address 10 times a day.