In networking, there are a few topics where trunks are used. One of them is when we use VLANs (Virtual LAN) between switches. The 802.1Q protocol is used to tag the VLAN information on the Ethernet frames. For example, between two switches we use the 802.1Q protocol, this link is called a 802.1Q trunk. You will also see trunks when talking about SIP (Session Initiation Protocol). SIP is a protocol for establishing, maintaining and ending VoIP calls. A SIP trunk is similar to a an analog phone line, but then for VoIP.