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