There's no need to bookmark the IP. Sci-hub actually runs a DNS server and you can simply specify 31.184.194.81 as one of your DNS servers in your computer network settings. Under OS X you can just add it in Sys Prefs. It's probably similar under other OSes.
If you do that, sci-hub.org, sci-hub.io, sci-hub.club still work just fine.
If you do that, sci-hub.org, sci-hub.io, sci-hub.club still work just fine.