BT Business Hub 5 with Fixed IP address - "Router address" not being used

[kmbro]

I have a BT Business Hub 5 with 5 fixed IP addresses.  The email from BT tells me:-

User IP address range start = 81.137.xxx.241

User IP address range end = 81.137.xxx.245

Base IP address start = 81.137.xxx.240

Base IP address end = 81.137.xxx.247

Default Gateway = 81.137.xxx.246

Subnet mask = 255.255.255.248

Subnet size = 8

Obviously 81.137.xxx is really a number in BT's IP address pool.

I've read through Configure the BT Business Hubs and assign multiple Static IP addresses | BT Business and done the following:-

• Visit http://btbusinesshub.home in a browser
• On the Home tab, click Configure button for BT Static IP
• Enter admin password
• Set Static IP address = on
• Set Router address = 81.137.xxx.246
• Set Subnet mask = 255.255.255.248
• Set allow incoming ping = true
• Click apply
• (yes, I haven't mapped any Fixed IP addresses to Business LAN devices - I don't want to do that right now)

From my wifi-connected laptop I visit http://en.utrace.de/ (other IP address reporters are available) which tells me I'm visiting from 81.134.183.nnn.  This is in BT's address range, but it isn't my router address, 81.137.xxx.246, which is what I'm expecting.

I tried restarting the router.  No difference.

Am I wrong in expecting NATted devices on my LAN to appear on the internet from my router's fixed IP address?  If not, what do I need to do to make it happen?

I've discovered on the Advanced Settings > Static IP tab that there's a subtab "MultiNAT".  This allows me to assign one of the five 81.137.xxx.24[1-5] fixed IP addresses to a single device on my business LAN, but at the top of this list I see the 81.134.183.nnn BT dynamic address, not  81.137.xxx.246. I guess this is the nub of the problem. Do I have to switch the router off for an amount of time to allow the dynamic address to time out so that BT will assign me my fixed IP address?

Thankd for your help.

[markp · Accepted solution]

Hi there kmbro

With a range of static IP's, the way they work on a BT connection is you configure the router the way you have, this allows you to manually assign all 5 statics in your range to devices behind the router.  The router itself does not get one of the static IP's, it gets a dynamic peer IP.

If you are needing the router to have a fixed IT, you can downgrade to a single static IP which we would then auto assign to the router when it connects and logs into the line on the corrct details.

I hope this helps

Markp

[kmbro]

Hi markp,

Thanks for the info.  I was expecting the 81.137.xxx.246 address to be used as the router's IP address, and therefore as the NAT IP for devices that haven't been assigned one of the 5 static IPs in my BT Hub. If that isn't the way it works then fair enough, but it is a bit confusing...

If I go into Advanced Settings > Broadband > Routing on my BT Hub then 81.137.xxx.246/32 is right at the top of my routing table on interface br0, along with the 5 assignable static IPs and 192.168.1.0/24 (which is my internal LAN).

So if the .246 address isn't used as my router's address, what IS it used for?

[markp]

Hi kmbro

the .246 gateway address is a unsabale IP only used as a routing IP to tell the router what the static IP range is, to allow you to manually assign the IP's in the range to devices behind the router.

Markp

[kmbro]

Ahah!  I think I understand what's going on now: my Business Hub no longer has 1 LAN, it effectively has 2: the original 192.168.1.0/24 and a new 81.137.xxx.240/29, both residing on the br0 virtual interface.  The latter is the home of the .241 to .245 externally-visible IP addresses that are assigned to particular internal hosts (such as my MacBook), and on it the hub is .246 and is used as the default route by the other 5.  IP packets from the 5 hosts received on the br0 interface get forwarded to the pppX interface to head off to the outside world, and incoming packets follow the reverse path.

(Okay, incoming packets get forwarded for sessions that the .24x host initiated; others are dropped unless the Hub firewall is set to permit them).

So that's why the .246 address isn't visible outside - my assumption that it would be used as the NAT IP for DHCP hosts on the 192.168.1.0/24 LAN was wrong.