I installed gateway with NAT between PC and network. User of PC can chat via Lync client but can't call or join conference. What ports should be mapped via NAT for Lync client to use all Lync facilities? Updated: the question actually is about NAT port mapping of Lync client.

  • Is that an internal or an external user, did you use split DNS or not?
    – BastianW
    Dec 12, 2016 at 7:27
  • DNS is OK. User of pc.
    – Parfen
    Dec 12, 2016 at 17:10

1 Answer 1


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.

  • Firewall doesn't block any outbound connection to any remote address, ports. Likely NAT port mapping matters. All inbound connections will fail if there are no port mapping: gateway doesn't know which ip connect to within Lan created by gateway.
    – Parfen
    Dec 12, 2016 at 19:18

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