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. On PCs, 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. On microcontrollers, however, SLIP is still the preferred way of encapsulating IP packets due to its very small overhead.

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Both SLIP and PLIP have been replaced by increasingly-common networks, including home networking – and by other peer-to-peer connections such as USB, used to transfer files to a second computer where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.