54.235.199.197 although this is potentially subject to change (this is the IP address of bitcointalk.org)
The IP address of ip.bitcointalk.org is 45.55.191.107 but as above, this is potentially subject to change.
Thank you very much QS, it works now, 45.55.191.107 is the IP address of ip.bitcointalk.org, but it's weird I always get 54.235.199.197 from pinging.