I'm testing for SSRF vulnerabilities so my question is; can I check external IP (from another computer on the network) from a SSRF vulnerability?
I think SSRF vulnerability is actually used to gain information regarding the internal network and machines which otherwise are not accessible to outside world. Assuming the webserver which suffers from this vulnerability is inside a DMZ, then using SSRF you can perform port scan and know the status of ports and services running on those internal machines. When you say external IP(another computer on network) I assume you refer to other machine/host which is in the same network of that of the machine that host the webserver. In this case yes you can perform port scanning of that machine.