In a non-switched LAN, you simply need to observe the packets, find something with a source or destination MAC that is the one you want to talk to and parse the IP out. Most machines won't have anything listening for pure-ethernet connections, but you may still be able to map that MAC to an IP address. The payload is merely transmitted by TCP and "only the application cares".ĪRP is used when someone on the local network wants to send an IP packet, to an IP it doesn't have a MAC address for and is basically an ethernet-level broadcast saying "who has IP address blah?". The TCP header contains various control information, various flags (what part of the session is this, what's the sequence number, source and destination TCP ports.
The IP header contains various control information, like source and destination IP and the encapsulated protocol (TCP, in this example). The ethernet header contains various control information, including the source and destination MAC addresses on the LAN. It may also (and usually does) have a "default gateway" (that is, an IP address through which it sends all packets going to somewhere other than a local subnet).Ībstractly, a packet destined for a web port looks something like:
Many network interfaces, however, support. Each address can be stored in hardware, such as the card's read-only memory, or by a firmware mechanism. On the IP level, each machine has (at least) one IP, in (at least) one subnet. MAC addresses are primarily assigned by device manufacturers, and are therefore often referred to as the burned-in address, or as an Ethernet hardware address, hardware address, or physical address. IP addressing is at some level symbolic, usually relying on lower-level network protocols (with less hierarchic structure) to actually complete the transmission.
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