SLIP, Serial Line IP, is an early method for carrying Internet Protocol packets over serial links. SLIP provided dial-up telephone and other point-to-point serial connections using IP. It was used as a WAN access method for modems, serial ports, routers, and simple remote links before PPP became the more capable and widely preferred point-to-point protocol.
SLIP was important because it was simple. It gave early Internet users and network operators a practical way to run TCP/IP over ordinary serial lines and dial-up modems. A SLIP connection could turn a personal computer, workstation, or small router into an Internet-connected host at a time when full-time LAN or broadband access was not common.
What SLIP Does
SLIP is best understood as a framing mechanism. It takes an IP datagram and places special END characters around it so the receiver can identify packet boundaries on a serial byte stream. If the data itself contains special control characters, SLIP escapes them so they are not mistaken for frame delimiters.
RFC 1055 describes SLIP as a nonstandard but widely used method for transmitting IP datagrams over serial lines. The protocol does not define address negotiation, authentication, error correction, protocol identification, compression negotiation, or link-quality monitoring. It simply provides a way to mark where one IP packet ends and another begins.
Why It Was Useful
SLIP fit the early Internet because it was small, understandable, and easy to implement. That made it attractive for Unix systems, terminal servers, modems, routers, and early personal-computer Internet access software. A user with a dial-up account and the right TCP/IP software could connect to an Internet service provider and run applications such as Telnet, FTP, email, Gopher, Mosaic, and Netscape over IP.
The original 2003 page referenced the era when SLIP and PPP accounts were required for graphical Internet access. That history matters: before web access became ordinary, getting a computer "SLIP-ready" or "PPP-ready" could involve modem strings, serial-port settings, packet drivers, IP addressing, DNS configuration, and client software that many users had never configured before.
Limitations
SLIP's simplicity was also its weakness:
- No authentication: SLIP itself does not verify the user or peer.
- No address assignment: IP addresses and other parameters must be configured out of band.
- No protocol field: basic SLIP carries IP only, rather than identifying multiple network-layer protocols.
- No error detection: SLIP does not include its own checksum or frame check sequence.
- No link negotiation: both ends must already agree on behavior.
- No standard management: there is no built-in mechanism like PPP LCP for checking or controlling the link.
These gaps made interoperability and support harder. Two SLIP implementations could both be simple and still differ in operational assumptions. That is why PPP became the better long-term answer for dial-up and point-to-point IP links.
Compressed SLIP
Compressed SLIP, often called CSLIP, used Van Jacobson TCP/IP header compression to improve performance on slow serial links. RFC 1144 describes a method for compressing TCP/IP headers, reducing repetitive header overhead for interactive traffic such as Telnet. On low-speed modem links, saving header bytes could noticeably improve responsiveness.
Header compression did not turn SLIP into a full link-management protocol. It helped performance, but the basic limitations remained. PPP could also use header compression while adding negotiation, authentication, and multiprotocol support.
SLIP Versus PPP
PPP replaced SLIP in most environments because PPP handled the tasks operators needed for real service:
- LCP: link establishment, configuration, testing, and termination.
- NCPs: network-layer configuration such as IPCP for IPv4 and IPv6CP for IPv6.
- Authentication: PAP, CHAP, and other authentication methods depending on implementation.
- Multiprotocol support: the ability to carry more than just IP.
- Error detection: PPP framing includes a frame check sequence.
- Extensibility: options could be negotiated rather than assumed.
For users, the practical difference was that PPP accounts were easier for providers to manage and easier for customers to configure reliably. For network operators, PPP provided a service framework; SLIP provided only packet framing.
Where SLIP Still Appears
SLIP is mostly historical, but the idea still appears in a few places:
- Legacy systems: old routers, terminal servers, Unix systems, modems, and embedded devices.
- Embedded and lab environments: simple serial IP experiments where minimal code matters.
- Retrocomputing and restoration: connecting older systems to modern IP networks through serial ports.
- Protocol education: a clean example of how framing is different from routing, addressing, or authentication.
- Specialized gateways: serial-to-IP bridges or constrained devices where a minimal encapsulation is enough.
Modern production networks should usually prefer PPP, Ethernet, cellular IP, IPsec, WireGuard, TLS-based application transport, or a purpose-built industrial protocol stack depending on the environment. SLIP is useful when the goal is compatibility or minimal implementation, not strong security or rich operations.
Security And Operations
SLIP does not encrypt traffic, authenticate peers, assign addresses, or protect against link misuse. If SLIP is still used, treat it as a very low-level transport and add protection elsewhere. Put the serial endpoint behind a controlled access path, restrict what the connected host can reach, and avoid exposing SLIP-connected devices directly to untrusted networks.
Operationally, document baud rate, serial settings, IP addresses, routing, modem behavior, cabling, and recovery steps. Many SLIP problems look like IP failures but are actually serial-port, flow-control, cable, modem, or framing issues.
Design Lessons
SLIP's history is a useful reminder that the smallest workable protocol can accelerate adoption. It carried IP over serial lines at a time when that was exactly what many users needed. But it also shows why operations matter. As Internet access became mainstream, administrators needed negotiation, authentication, error detection, and centralized service control. PPP supplied those missing pieces.
In 2026, SLIP is a legacy and educational technology, not a new-network design choice. Its legacy lives in the simple idea that an IP packet can be carried across almost any point-to-point medium if both ends agree on framing, addressing, and behavior.