EC-11.4 Addresses and Routing
You can read an address and its prefix length, say which part names the network, follow a packet through a forwarding decision, describe how routes are learned, and explain what a router does when a packet is too large or has been travelling too long.
This is the layer that makes a network of networks possible, and it is where the internet's actual design decisions become visible. It is also the topic that pays off most directly in ordinary work: almost every practical connectivity problem a graduate meets is an addressing or routing problem, and the ones that are not are usually resolved by ruling those out first.
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Hierarchical addresses, and why they beat flat ones
An address that encodes where a machine is lets a router hold one entry for a whole region. A flat address would need an entry per machine, which no router in the world could store.
NPTEL: Computer Networks and Internet Protocol (IIT Kharagpur) · CoursePrefixes, masks and what a subnet actually is
A prefix length says how many leading bits name the network and how many are left for machines within it. Splitting a range into subnets is choosing where to put that boundary.
NPTEL: Computer Networks and Internet Protocol (IIT Kharagpur) · CourseForwarding: one decision, made per packet
A router compares the destination against its table and picks the entry with the longest matching prefix. That single rule is what makes a specific route override a general one.
NPTEL: Computer Networks (IIT Indore) · CourseHow routes are learned, in two families
One family has routers tell their neighbours what they can reach and at what cost. The other has every router learn the whole map and compute its own shortest paths. Both are in use, at different scales.
NPTEL: Computer Networks and Internet Protocol (IIT Kharagpur) · CourseFragmentation, lifetime and error reporting
A packet too large for the next link must be split or refused, and a counter that decreases at each hop stops a packet circling forever. A companion protocol reports both back to the sender.
NPTEL: Computer Networks (IIT Indore) · CourseAddress translation, and the internet as it actually is
Address shortage led to private ranges rewritten at a border, so most machines are not directly reachable. This breaks the original end-to-end design and explains many awkward connectivity problems.
NPTEL: Computer Networks and Internet Protocol (IIT Kharagpur) · Course
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