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Cisco® Field Manual: Catalyst® Switch Configuration by Steve McQuerry, Dave Hucaby

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B-3. Well-known IP Port Numbers

Transport layer protocols identify higher-layer traffic with 16-bit fields called port numbers. A connection between two devices uses a source and a destination port, both contained within the protocol data unit. The User Datagram Protocol (UDP) header format is shown in Figure B-4 with the source and destination port fields shaded. The UDP checksum is optional for IPv4. Figure B-5 shows Transmission Control Protocol (TCP) header format with the source and destination port fields shaded.

Figure B-4. UDP Datagram Format Showing Port Fields

Figure B-5. TCP Segment Format Showing Port Fields

Both UDP and TCP port ...

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