I'm fully aware of how DHCP works, I told him what I thought was actually useful to him.

In reply to a post by MrSaffron:

An IP tunnel (VPN) service could possibly achieve what you want, but to be reliable would require a business grade VPN router at your end.

Problem there when Sky currently require you to use their router. Maybe a Billion or Draytek with the Sky supporting firmware *might* do it, unlikely though.

One of the many dynamic DNS providers is much more likely, run the client on the server.

The Billion 7800N has built in support for Dyn (which is what Dynamic DNS is now called) as well as other services, it also supports MAC Encapsulated Routing (MER) which Sky use for authentication (though you need to get the MAC of the Sky router they send you) and also has a 100 Meg EWAN port so can be used with FTTC.

The vpn router need not terminate the adsl or fibre line, but being behind a nat router can complicate things.

In reply to a post by MrSaffron:

but being behind a nat router can complicate things.

Very much so, unless you're a networking guru

No-IP is a dynamic DNS service. My ADSL router can automatically registers with this service each time my external IP address changes.

However instead of using the router to perform this registration function, I can use Linux software. The Linux software does not know when the external IP address changes, it just re-registers on a regular basis so that if the IP address does change the DNS entry is changed to point to the new address.

No-IP is one of the supported DDNS services on the 7800N IIRC.