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Ports And Protocols Used By Skype for Business/Lync Secure Chat And Conferencing

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

PortProtocolNotes
 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/TCPPort range used for inbound and outbound media transfer through the external firewall 
 6891-6901 TCPPort 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.

Ref: https://kb.intermedia.net/Article/1679