The Serial Line Internet Protocol (SLIP) is a mostly obsolete encapsulation of the Internet Protocol designed to work over serial ports and modem connections. It is documented in RFC 1055 (http://www.ietf.org/rfc/rfc1055.txt). SLIP has been largely replaced by the Point-to-Point Protocol (PPP), which is better engineered, has more features and does not require its IP address configuration to be set before it is established.

SLIP modifies a standard Internet datagram by appending a special "SLIP END" character to it, which allows datagrams to be distinguished as separate. SLIP requires a port configuration of 8 data bits, no parity, and EIA or hardware flow control. SLIP does not provide error detection, being reliant on other higher-layer protocols for this. Over a particularly error-prone dial-up connection therefore, SLIP on its own is not satisfactory.

A version of SLIP with header compression is called CSLIP (Compressed SLIP).

The Parallel Line Internet Protocol (PLIP) is very similar to SLIP, but works at higher speeds via a parallel port.

Both SLIP and PLIP have been replaced by increasingly-common networks, including home networking – and by other peer-to-peer connections such as USB, used to transfer files to a second computer where a network is not necessary or available.

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