Memory hook: Name the layer, then name the job.
Must remember
| OSI layer | Remember |
|---|---|
| 7 Application | Protocols used by applications, such as HTTP and DNS. |
| 6 Presentation | Data representation, encoding and related transformation concepts. |
| 5 Session | Dialog/session coordination concepts. |
| 4 Transport | TCP/UDP, ports and end-to-end transport behavior. |
| 3 Network | IP addressing and routing between networks. |
| 2 Data link | Local frames, MAC addresses, switching and VLANs. |
| 1 Physical | Signals, media, connectors and bit transmission. |
The TCP/IP model groups functions differently; real protocols do not always fit neatly into one textbook box. Encapsulation adds headers as data descends the stack. On a routed path, link-layer addresses change at each link; the IP destination normally remains the end host unless translation/tunneling changes it.
TCP establishes a connection, sequences bytes, acknowledges delivery and handles retransmission/flow control. UDP sends datagrams without those built-in guarantees; an application protocol can add its own reliability. TCP reliability does not mean a business transaction executed exactly once.
A switch learns source MAC addresses and forwards frames within its Layer 2 domain. A router selects next hops between IP networks. A multilayer switch can do both. Firewalls enforce traffic policy; proxies terminate/mediate connections; load balancers distribute service traffic. NAS provides file access, while a SAN commonly presents block storage over a storage network.
Unicast addresses one recipient; broadcast a local IPv4 broadcast domain; multicast a group; anycast the routing-selected instance of a shared address. IPv6 uses multicast rather than broadcast. Routers normally separate broadcast domains.
Star, mesh, hub-and-spoke, spine-leaf, three-tier and collapsed-core designs trade cost, fault paths and scale. A logical topology and a physical cabling diagram describe different views.
Choose under exam pressure
| Requirement | Choice and reason |
|---|---|
| Forward between IP subnets | A routing function. |
| Share files with clients | NAS/file service rather than raw block storage. |
| Reach a nearby service instance using the same address | Anycast with appropriate routing. |
Traps
- A switch can implement several layers; identify the function being tested.
- UDP is not automatically inappropriate for reliable applications.
Active recall
1. What does a switch normally learn from an arriving frame?
Its source MAC address and incoming port.
2. What changes at each routed Ethernet hop?
The local frame’s source/destination MAC addresses.
3. Does IPv6 use broadcast?
No; multicast and other mechanisms serve those purposes.
4. NAS versus SAN?
NAS typically exposes files; SAN typically exposes block storage.
5. Why distinguish bandwidth and application success?
Transport delivery does not prove that the application processed a business operation correctly.