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82 posts

Master Geek


Topic # 151770 4-Sep-2014 15:16
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Hello, if I have a bunch of printers with static IP addresses, and I establish a new DHCP server, how does the DHCP server realise that those static IP's are already in use ?
Does the DHCP server send out a query on the network before it allocates addresses ?

Thanks for any info.

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Master Geek
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  Reply # 1121904 4-Sep-2014 15:19
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Nope, the DHCP server will not take any notice - you will need to reserve those addresses from the new DHCP scope otherwise you'll get dupe IP issues.


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  Reply # 1121907 4-Sep-2014 15:20
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If you are using the same operating system and DHCP software, you should be able to export and import the settings.

Some DHCP Servers can be configured to test (ping) an IP address before handing it out.

Otherwise, just enter these static IP addresses as a Reservation or an Exclusion manually into your DHCP software.

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  Reply # 1121908 4-Sep-2014 15:21
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We've just covered DHCP servers in one of my lectures this week actually :D

You'd simply add the range of static IPs currently issued, into an Exclusion on the DHCP server. That way it won't issue those IPs to anything. You do the same thing to ensure servers and the like have a static IP.

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  Reply # 1121946 4-Sep-2014 15:48
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initially the dhcp server wont know that the ip addresses have been assigned statically to a device. it will offer the address to the client that that is requesting an address. the client should then test this address and if it finds it is in use it will send a DHCPDECLINE message back to the dhcp server. the client will then start the request for an ip address again. the dhcp server should then mark the declined ip address as a bad address and not try and give it out anymore

this is how it should work in theory.

you could make life simpler by setting the start of range of the addresses being handed out above the ip addresses of the printers. this will also allow you to manually assign other ip addresses in the low range you know is always available

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  Reply # 1122051 4-Sep-2014 18:20
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Either put your static devices outside of the DHCP scope (but still within the subnet), or create reservations. Nice thing about reservations is that if you reset the configuration on those printers, itll just jump straight back on the right IP.

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Ultimate Geek
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  Reply # 1122063 4-Sep-2014 18:42
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If you have to use static IP's for your printers it's probably better that you set them up in your DHCP server (mapping via mac address) instead of hard-coding on each individual printer.
Otherwise, would be a wee bit of an administrative nightmare I would've thought.

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  Reply # 1122112 4-Sep-2014 19:53
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some DHCP servers have the option to test if an address is already in use before it hands out an address that would conflict.  Windows Server DHCP comes to mind.

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Ultimate Geek
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  Reply # 1122130 4-Sep-2014 20:36
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nathan: some DHCP servers have the option to test if an address is already in use before it hands out an address that would conflict.  Windows Server DHCP comes to mind.

Circa Windows Server 2008 yes :-)

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