OSI teminology: ES (end system) is used as host in TCP/IP, IS( Intermediate System) is used for a router
In network layer of OSI reference model, there are 2 modes for end-to-end communication: connectionless-mode and connection mode. For connection-oriented mode, an adaptation of the x.25 is used, there is no analogous connection-oriented network layer protocol in TCP/IP. Connectionless-mode network protocol (clns) is to OSI networks what IPv4/v6 are to TCP/IP networks.
NSAP address is the address used in OSI reference model, unlike ip address, each IS node has only one NSAP address, each interface on the IS node is represented with local circuit ID. NSAP has minimum 8 octets, with only AFI, system ID and SEL. Tha maximum size is 20 octotets
Levels of Routing in OSI networks:
Level 0 routing: between ES and IS
Level 1 routing: between ES nodes within the same area
Level 2 routing: between ES nodes which are in the different area of the same domain
Level 3 routing:between ES nodes which are in different domains
IS-IS provice level 1 and level 2 routing. BGP for inter-autonomous system routing in TCP/IP is fairly analogy of Level 3 routing.