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Contributor
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Message 1 of 10

DHCP Address Allocation

Over the years , having had many versions of the BT hubs, I have noticed that each time a new hub is used, the DHCP table does not reset back to a ‘clean slate’ as I would expect, with addresses starting from 64 going upwards, but that the devices somehow seem to carry their previous address from the old hub forward so that they have the same addresses in the new one. Also, once used, and then vacated, maybe because the device is obsolete or no longer used, or was a visitor,  an address never seems to be re issued to another new device, even when a new hub is brought into service. The end result is that now all the allocated addresses on my smart hub are up in the high hundreds, with none at the low end.  Is there any way to reset the clock and get everything back to the low end of the range? I have set the ‘always use this address’ for some devices, after it has been allocated by DHCP, but does this carry over to the next hub?

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Distinguished Sage
Distinguished Sage
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Message 2 of 10

Re: DHCP Address Allocation

The only way to clear the DHCP table in the home hub, is with a factory reset, however any device which has retained its IP address, will most likely get the same one again.

If you execute this command on any Windows devices, before you connect them to the home hub, then they should request a new IP address when you connect them.

ipconfig /flushdns   (there is a space after ipconfig)

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Aspiring Contributor
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Message 3 of 10

Re: DHCP Address Allocation

Keith

I think you meant to suggest the following.

Ipconfig /release

Followed by

Ipconfig /renew

This will release and then renew the ip address for the device.

Flushdns will just clear the dns table.

 

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Distinguished Sage
Distinguished Sage
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Message 4 of 10

Re: DHCP Address Allocation

Yes, that would make more sense, I was looking at one of my older posts on this forum.

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Contributor
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Message 5 of 10

Re: DHCP Address Allocation

I have no Windows devices, all Apple and other stuff.  But why don’t the unused low addresses get used for new devices, the hub just seems to add them on at the end of the list, as it were.

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Distinguished Sage
Distinguished Sage
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Message 6 of 10

Re: DHCP Address Allocation

Why does it matter? An IP address is an IP address. As long as there are no conflicting addresses I don't see a problem. If you are that bothered, set a static address on your devices themselves and don't use DHCP.

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Contributor
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Message 7 of 10

Re: DHCP Address Allocation

Liquorice, you are right, it doesn’t really matter, I just like to see some order and to know how these things work.

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Distinguished Sage
Distinguished Sage
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Message 8 of 10

Re: DHCP Address Allocation


@Bramshot wrote:

Liquorice, you are right, it doesn’t really matter, I just like to see some order and to know how these things work.


You will never know how things work in BT world other than it is different from the real wold. Smiley Very Happy

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Distinguished Sage
Distinguished Sage
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Message 9 of 10

Re: DHCP Address Allocation

Fair enough. Unfortunately when it comes to Home Hubs its all a mystery how they work!! Smiley Happy

 

Typing as gg posted!

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Contributor
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Message 10 of 10

Re: DHCP Address Allocation

Thanks for your replies. Believe me, I am well versed in the idiosyncrasies of BT hubs, having now had them all except 4 and 5, which somehow passed me by. I used to be a major contributor to one of the forums on hub problems in my ADSL days, but all my problems disappeared overnight when I changed to Infinity / Hub 3, so I dropped out of that. It seems nothing much has changed in my absence.

I am guessing the hub DHCP logic is something like;

Hub: Hi there device, do you already have an IP address?

Device: Either, ‘Yes, it’s nnn.nnn.n.nn’ or ‘No’.

Hub: ‘ok that address is in range, you can keep it’, or that address is out of range, use this new one’, or ‘use this one’.

That seems to account for the behaviour I have seen.

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