The following table summarizes the ports and protocols used by Skype for Business/Lync clients:

Port Protocol Notes
 5061  TCP (SIP) Used for SIP/MTLS authentication of A/V users. Communications flow outbound through the internal firewall 
 443  TCP (HTTP) Used by Skype for Business clients/Lync connecting from outside the intranet for SIP communications 
 1024-65535  UDP/TCP Port range used for inbound and outbound media transfer through the external firewall 
 6891-6901  TCP Port range used by Skype for Business/Lync clients for file transfer

If you experience problems with Audio/Video Conferencing, media or file transfer, please verify that corresponding ports are open on your firewall.