We have an IBM server with ESXi 5.0 on it. The guests are able to see the gateway and work just fine but we are unable to connect to the management network at all outside its own subnet. Running network tests fails on all tests. All the settings look correct to me but for some reason it just fails. I will probably just upgrade to 5.1U2 eventually but if there is a quick fix that would be good. I have tried resetting the network, re adding the gateway etc.
If the Management Network is accessible from the (same) network, then you may want to triple check the current settings to ensure the default gateway as well as the subnet mask are configured properly (e.g. no typos). If the host is behind a firewall, you may also want to check the firewall rules.
I can confirm that the IP, subnet mask and gateway addresses are all correct and set the same as the guests.