Lukor,

Yes, it showed the reverse IP address as mentioned in the original post.

But I think we have solved the problem. It was not Avast, but it was their DSL Router.
I have now replaced the DSL Router with a Netgear N300.

The previous one was a Belkin F5D8636-4 v2 router (only 2 months old).
I must say, I am getting less impressed with Belkin.