Mongodb is installed on machine A with IP 192.168.51.84;
I can connect to A with putty from machine B with IP 192.168.70.1, but I cannot access mongodb on A with robomongo.
Mongo program on A has been started and can be accessed with Mongo command on A.
There is no problem accessing the mongodb server on the extranet.
Machine b is a windows 8 system and machine a is red hat 6.
If it is the RPM or Deb installation version of MongoDB 2.6, only the local port will be monitored by default. Look at the settings of bind_ip in your mongod.conf file.
Also check your firewall port to see if Mongo port is not allowed