Link layer and IP

คำศัพท์

English	ความหมาย
MAC address/mæk əˈdres/	A link-layer address associated with a network interface.
CRC	Cyclic redundancy check: an error-detection value calculated from transmitted bits.
TTL	The IP hop-limit field that routers reduce to stop packets circulating indefinitely.
IP address/ˌaɪ ˈpiː əˈdres/	An address identifying an interface within an IP network.

English

Two addresses, two scopes

An Ethernet frame: 6-byte destination MAC address, 6-byte source, 2-byte type, payload, 4-byte CRC. The MAC names a neighbour and changes every hop.
An IPv4 header: version/IHL, total length, TTL, protocol, checksum, 4-byte source and destination IP address. The IP names the stranger and never changes.

{exercise:uni-cs0902-scopes}

ฝึกฝน

Crossing 3 routers, the destination IP —

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English

45 00 00 3C … 40 06 …: version 4, IHL 5 — $5 \times 4 = 20$ bytes of header; total length $0x003C = 60$ (20 header + 40 payload); TTL $0x40 = 64$; protocol 6 = TCP.
The CRC protects the whole frame link by link; the IP checksum protects only the header end to end — routers rewrite the header (TTL decrements) but not the data, so integrity is scoped to what its rewriter touches.

{exercise:uni-cs0902-decode}

ฝึกฝน

IHL = 5 means the header is —

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English

Crossing 3 routers, a packet rides 4 frames and carries 4 different destination MACs — one per hop, chosen by IP — while its destination IP stays fixed the whole way.
IHL counts 32-bit words, total length counts bytes: mixing the two units is the classic header bug.

{exercise:uni-cs0902-units}

ฝึกฝน

Total length 0x003C with IHL 5 leaves — payload bytes

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