Internetworking connects heterogeneous networks that may use different technologies, protocols, and addressing schemes.
Challenges:
- Different protocols and frame formats
- Different addressing schemes (IP vs MAC vs others)
- Different MTUs (Maximum Transmission Units)
- Security policy mismatches
- QoS differences
Interconnection Methods
Networks are interconnected at different OSI layers. Higher-layer devices understand more of the packet and can translate between more dissimilar networks.
| Layer | Device | Function |
|---|---|---|
| Physical | Repeaters, hubs | Amplify signal; no protocol awareness |
| Data Link | Bridges, switches | Forward by MAC address; same protocol only |
| Network | Routers | Forward by IP address; connects IP networks |
| Transport | Transport gateways | Translate between transport protocols |
| Application | Application gateways | Full protocol translation (e.g., email relay) |
Fragmentation
When a packet crosses a network with a smaller MTU, it must be fragmented. The IP header carries offset and more-fragments flag to allow reassembly at the destination.
- IPv4: fragmentation by any router along the path.
- IPv6: fragmentation only at the source (routers drop oversized packets and send ICMPv6 Packet Too Big).
Path MTU Discovery (PMTUD) avoids fragmentation by detecting the minimum MTU along the path before sending.
Tunneling
A packet from one protocol is encapsulated inside another protocol’s packet to cross a network that does not support the original protocol.
Example: IPv6 packets tunneled inside IPv4 (6in4) to traverse IPv4-only segments. The IPv6 packet becomes the payload of an IPv4 packet; the receiving tunnel endpoint strips the IPv4 header and delivers the IPv6 packet.