Hi @grenzo!
This switch may have many IP addresses - one or more per Vlan-interface. And also it may have just VLANs without SVI (Vlan-interface), so it doesn't work like this - plug a cable to arbitrary port and access the switch. Also, even if you are lucky enough and the port you are connected on is Access and its VLAN has Vlan-interface, it doesn't mean it can't have an ACL that blocks connection from clients in this VLAN to the switch itself allowing only transit traffic. Also, even if the switch allows VLAN traffic to reach its IP of the Vlan-interface, it doesn't mean you can telnet or ssh to it. You may be able to, but both protocols should be explicitly configured.
Generally speaking you could use a network scanner like Nmap and try to scan the whole local subnet for active hosts, then find each host's MAC and compare it with the MAC you already know, but for this task you need to know the subnet you are in. If you decide to go this way - https://nmap.org - they have very devent documentation. Also since it's a free and quite popular tool you won't have issues finding help for your scanning needs.
For me the only valid way to find out IP addresses that belong to your switch is console connection to the switch. And then you can do whatever you need - look for IP addresses by "display ip interface brief", change configuration, whatever.
How can you run any command if you don't know the IP of the switch, therefore you can't establish a CLI session over a telnet or ssh and therefore you can't run any command?
Sorry, but let's clarify the situation. Do you want to say that you don't want to find an IP address of the switch itself, but you want to find an IP of a client connected to the switch? Because your initial message is quite confusing....
JG236A is a module for 7500 switch. Your PC and 5120 (JG236A) are connected to the same module... Ok, so one way to find an IP address by MAC address is to look in ARP table:
display arp all | include <mac_address>
MAC address should be in the following format: xxxx-xxxx-xxxx.
If you won't find anything, it will mean that 7500 either doesn't have Vlan-interface in the VLAN where 5120's IP resides or 7500's Vlan-interface doesn't get anything from the 5120 by ARP.
Another way - 5120 may announce to 7500 its management IP address by LLDP, so use this command to find it out:
display lldp neighbor-information
or if you know the port on the 7500 where 5120 is connected, you can use more precise command:
display lldp neighbor-information interface <port_number>
Hope this helps!
Bad luck, your switch does not report its management IP. Then ARP table is your only hope.