from ntl help:
Is my IP address Dynamic or Static?
It is dynamic and although in general use it does not change frequently, ntl:home make no guarantees on how long you will keep an IP address.
How do I get a Static IP address? I need one to run my server!
ntl do not provide static IP addresses. In addition, if you plan to run a server you must ensure it is in line with the Broadband Service User Policy, which can be found here.