Then at the protocol level the n/w may decide not to award them a block if they have X% of the total hash power.
And how would you know how much hash power they have?
There are only 2 ways to get information about how much hash power a pool has.
1. They tell you. You can't exactly trust a pool to tell you honestly how much hash power they have if they are trying to use that hash power to pull off an attack.
2. You count up the number of blocks they recently solved, and calculate an estimate based on the total blocks the network as completed in that time. (If 144 blocks are solved in a day, and someone solved 72 of them, then they have 50% of the hash power). You can't trust a pool to use the same bitcoin payout address for every block they solve or broadcast it from the same IP address every time if they are trying to hide what they are doing.
How come a pool can continuously change the public IP ? Wont the miners lose the point of contact then ? I mean, they need to know the IP and port for mining ...is not it ?