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Changing IP Address

Phil_R89 — Sun, 07 Aug 2022 20:30:45 GMT

Hi,

I'm trying to setup my network with static IP addresses I have selected a few variations without success. My most resent attempts were 10.5.25.1 and 172.5.25.1 both with a subnet mask 255.255.255.0 I keep getting the message below.

- The DHCP start IP address is not valid.

I cannot find a list or range of valid IP address. I have a BT smart Hub 2. Please can somebody help?

Thank you,

Phil

Re: Changing IP Address

licquorice — Sun, 07 Aug 2022 20:58:37 GMT

Is there a particular reason not to use the default 192.168.1.xxx schema?

172.5.xxx.xxx isn't a private address range in any case

Re: Changing IP Address

Phil_R89 — Sun, 07 Aug 2022 21:07:00 GMT

I'm just starting to learn about network administration.

I don't have anything against 192.168.1.xxx in particularly, other than a vague and possibly incorrect belief that it's generic nature can reduce security slightly. I though it was possible to use any IP address within a private network and figured I would try something different as a exercise. I've just finished building a domain server and wanted get my network architecture in order before setting up Active Directory.

Re: Changing IP Address

Les-Gibson — Sun, 07 Aug 2022 21:20:13 GMT

You might find this resource useful.

IP Addressing - Tutorial (vskills.in)

Re: Changing IP Address

Anonymous — Sun, 07 Aug 2022 21:51:57 GMT

It's not entirely clear, at least to me, what you're trying to achieve.

The Hub has a gateway of 192.168.1.254 & the DHCP pool starts at 192.168.1.64 (I think!). So if you want to assign static addresses to devices, they need to be in the range of 192.168.1.1 - 63.

If you're trying to change the gateway address of the Hub then maybe you need to also change the DHCP pool to match. I don't use a Hub so no idea if it can pick up the DHCP range from the gateway or if it needs to be configured manually.

Edit

As @licquorice said, the address chosen also needs to be within the privates ranges or you risk conflicting with an external device somewhere on the WWW. Again I don't know if the Hub is smart enough to refuse to save a non-private address.

Re: Changing IP Address

licquorice — Sun, 07 Aug 2022 21:35:09 GMT

As said, you need to set Gateway address for the hub , DHCP pool start and finish range and then set static addresses on devices outside of that range.

As far as I'm aware, you can use any schema on the SH2 within the Class A,B or C ranges.

Class A IP addresses. Configurations range from 10.0. 0.0 to 10.255. 255.255
Class B IP addresses. Configurations range from 172.16. 0.0 to 172.31. 255.255
Class C IP addresses. Configurations range from 192.168. 0.0 to 192.168. 255.255

There is absolutely no difference in the level of security whichever schema you choose.

Re: Changing IP Address

Phil_R89 — Sun, 07 Aug 2022 22:05:40 GMT

I may be mistaken and would be grateful if anyone can help me to understand further.

I thought:

- BT gave me a public IP address (ie 24a0:3dc2:a01b:c621:a112:51ba:e2b0:11e)

- BT also gave me hub with a default private IP address of 192.168.1.254. This address isn't seen by the internet and I had believed it could be anything because only devices within my network would use it.

- this could provide a small reduction in security as anyone with a Wi-Fi enabled device in my vicinity will know Hub Gateway IP Address is 192.168.1.254)

The below configuration did not work but does fit into the Class A IP addresses. Configurations range from 10.0. 0.0 to 10.255. 255.255

Re: Changing IP Address

Les-Gibson — Sun, 07 Aug 2022 22:22:47 GMT

Not all ISP's use the same gateway address so others would have to know which supplier you were with to guess your gateway address and then of course they would need to know your wi-fi password.

Re: Changing IP Address

Phil_R89 — Sun, 07 Aug 2022 22:29:19 GMT

True but the default Wi-Fi name is publicly broadcast as BTWholeHome, so the only thing really stopping a person for accessing your network is their ability to brute force your password.

Re: Changing IP Address

Les-Gibson — Sun, 07 Aug 2022 22:37:14 GMT

A brute force attack can take minutes or it can take ages, it all depends on the strength of your password, are you worried about a neighbour trying to hack you?

Re: Changing IP Address

Phil_R89 — Sun, 07 Aug 2022 22:43:32 GMT

I've managed to set my IP address, it seems as though you can have any IP address within the Class B or C ranges defined by liquorice earlier:

Class B IP addresses range 172.16. 0.0 to 172.31. 255.255
Class C IP addresses range 192.168. 0.0 to 192.168. 255.255

Thank you to everyone for helping.

Re: Changing IP Address

Phil_R89 — Sun, 07 Aug 2022 22:54:13 GMT

Please can anyone confirm or correct my understanding of public and private IP addresses?

If BT choose to set their own classes and limit the choice of private IP addresses, that's fine, I'm sure they have a perfectly coherent business case for it.

Am I correct in thinking that in theory a private IP address can be anything within the limitations of IPV4?

Re: Changing IP Address

licquorice — Mon, 08 Aug 2022 06:59:15 GMT

No, the address ranges I listed are the only ones that are designated as private addresses and are not routable on the internet.

Re: Changing IP Address

licquorice — Mon, 08 Aug 2022 10:41:41 GMT

@Phil_R89 I can fully recommend 'Computer Networks' by Andrew S Tanenbaum as a good introduction to networking. I believe the latest edition available is Fifth Edition.

Re: Changing IP Address

Jayam — Tue, 07 Mar 2023 14:45:26 GMT

Hi all,

after much faffing around and a number of useless call to BT I managed to find the answer.

Once you change the local IP address you need to update the DHCP settings - I suspect you already tried this (like me) and found it can't be changed either. Bad UI design if you ask me as it's not obvious how this setting works!

Basically, the trick is to select 'Custom' in the DHCP options list. This will allow you to change the DHCP address range to one that matches you new IP. Now, when you click 'Save' the new settings will be applied. It seems that if the DHCP address range is unchanged the settings are not saved (for good reason).

Hope that helps.