BT Shares its IP Addresses
IP address translation/sharing
What is it?
There are only a certain number of IP addresses and due to the expansion of the internet, these are running out worldwide. Eventually they'll be replaced by a new version. Until this time, addresses will need to be shared to ensure everyone can access a reliable broadband service.
We're preparing for this transition period by gradually introducing IP address translation/sharing whilst continuously testing and learning. The change is being introduced to BT Total Broadband Option 1 customers.
If you're sharing an IP address, your internet traffic goes through an extra translation step in our network before connecting to the internet to change it from a private address to a common public one.
Hope this may through some light.
Basically if you block anyone on a Shared IP you will block other users too.
I may not of explained myself some time back when I mentioned IPs being block allocated
but this is what I was trying to get across.