<P> The initial version of the 1822 protocol was developed in 1969: since it predates the OSI model by a decade, 1822 does not map cleanly into the OSI layers . However, it is accurate to say that the 1822 protocol incorporates the physical layer, the data link layer, and the network layer . The interface visible to the host system passes network layer addresses directly to a physical layer device . </P> <P> To transmit data, the host constructs a message containing the numeric address of another host on the network (similar to an IP address on the Internet) and a data field, and transmits the message across the 1822 interface to the IMP . The IMP routes the message to the destination host using protocols that were eventually adopted by Internet routers . Messages could store a total length of 8159 Bits, of which the first 96 were reserved for the header ("leader"). </P> <P> While packets transmitted across the Internet are assumed to be unreliable, 1822 messages were guaranteed to be transmitted reliably to the addressed destination . If the message could not be delivered, the IMP sent to the originating host a message indicating that the delivery failed . In practice, however, there were (rare) conditions under which the host could miss a report of a message being lost, or under which the IMP could report a message as lost when it had in fact been received . </P>

Where were the first 4 interface message processors (imp's) located