[toast0]

For wired Ethernet, a (decoded) frame starts with a 7-octet preamble of alternating 1 and 0 (0x55), then a "start of frame delimiter" which continues the alternation, except the last bit is 1 instead of zero (0xD5 -- least significant bit is sent first). Then the next 6 octets are the destination mac address. If the NIC is in normal mode, it will wake up when it sees a preamble, use the alternating pattern to synchronize its clock, wait for the start of frame delimiter (which is really the two 1 bits together -- in case the wakeup was a little slow); and then compare the destination mac address to its own mac address one bit at a time. The least significant bit of the first octet of the mac address indicates if it's a broadcast/multicast or intended for only one station; this is the first bit sent on the wire, so that the adapter knows to not do the normal comparison right away. Once the adapter determines the destination address doesn't match, it can stop processing the line. Ethernet requires 96 bits of idle between frames (interframe gap), which it can detect to rearm the preamble trigger.

Different versions of ethernet encode the logical signals differently, so the adapter may decode multiple bits at the same time (100BaseTX transmits in 4-bit groups, 1000BaseT in 8-bit groups), so preamble detection and address comparison would likely be a little different, but the same idea applies.

The Ethernet frame page [1] might be a helpful place to look for more info.

Also, as mschuster91 pointed out, an ethernet switch that can do cut-through switching [2], will be able to avoid buffering the whole packet to decrease latency; although it will need to have some capacity for buffering whole packets anyway -- if it's already sending a packet on the destination port, cut through switching isn't appropriate. An IP router can also do cut-through switching, once the IP header with destination comes in (or as much addressing as required for hashing among multiple links, if the destination is link aggregated)

[1] https://en.wikipedia.org/wiki/Ethernet_frame [2] https://en.wikipedia.org/wiki/Cut-through_switching