When referring to the layers of the TCP/IP network stack, I will use the abbreviations L2, L3, and L4 to refer to the
transport layers, respectively. The numbers are based on the famous (if not exactly current) seven-layer OSI model. In most cases, L2 will be a synonym for Ethernet, L3 for IP Version 4 or 6, and L4 for UDP, TCP, or ICMP.
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