MacIP refers to a standard for encapsulating Internet Protocol (IP) packets within the AppleTalk DDP protocol. This allows Macintosh computers with LocalTalk networking hardware to access the normally Ethernet-based connections for TCP/IP based network services. This was an important bridging technology during the era when Ethernet and TCP/IP were rapidly growing in popularity in the early 1990s.
Software implementing MacIP, such as MacTCP or Open Transport, was installed on the computer and a MacIP Gateway was placed elsewhere on the network. Applications that communicate with TCP/IP (such as Telnet) have their IP packets encapsulated in DDP for transmission across the LocalTalk network to the MacIP Gateway. The MacIP Gateway strips off the DDP encapsulation and forwards the IP packet on the IP network.
The gateways were often implemented as part of a LocalTalk-to-Ethernet bridge device, small hardware systems primarily designed to allow communications between LocalTalk and EtherTalk equipped AppleTalk machines (like the Mac II and a LaserWriter). MacIP routing was often implemented as an optional adjunct to the AppleTalk routing. Cisco Systems supported AppleTalk in their proprietary IOS (up to and including version 12.4(15)T14, on select platforms) which in turn could provide MacIP-Services.