Glossary
Link-layer type (LinkType / DLT)
The link-layer type is the number in a capture file that says how each packet's first bytes are framed: Ethernet, Linux cooked, raw IP, loopback and so on.
The link-layer type (LINKTYPE, often called DLT after libpcap's data link type) is a number stored in a capture file that tells the reader how to decode the start of each packet. Common values are 1 for Ethernet, 0 for BSD loopback, 101 for raw IP, 113 and 276 for Linux cooked capture (SLL and SLL2), and 127 for 802.11 with a radiotap header.
In pcap there is one link type per file; in pcapng each interface has its own, which is why some pcapng files cannot be converted to pcap without splitting them. A viewer that does not support a file's link type cannot decode its packets. The values are listed in the IETF LinkType registry draft referenced by the pcap specification. See how to open a pcap file online for the types PCAP Parser decodes.