For example, if you know the packets are actually raw Ethernet (Type 1):
You created a pcap file with a new version of tcpdump or Wireshark (which supports exotic DLTs) and are now trying to read it with an older version of libpcap or a legacy tool (e.g., an old tcptrace or a deprecated ngrep ). The old library simply has no entry in its switch-case statement for "276." -pcap network type 276 unknown or unsupported-