Practice: Placing Ten Protocols and Devices on the Stack

HTTP, DNS, TCP, UDP, IP, Ethernet, a switch, a router, a hub and a load balancer, each to its layer.

Placing protocols and devices on the OSI model is the first step whenever a network fault has to be located, and it is a skill that comes from practice. For each item, give its OSI layer number and say in a few words why: for a protocol, which job it does; for a device, which header it reads to decide where data goes.

HTTP
DNS
TCP
UDP
IP
Ethernet
A switch
A router
A hub
An application load balancer that routes /api to one group of servers and everything else to another

Show the layers and the reasons

Each protocol or device, its OSI layer, and the reason
Item	Layer	Reason
HTTP	7	Carries requests and responses between programs.
DNS	7	A program asks it a question and gets an answer; it is an application that other applications use.
TCP	4	Delivers a reliable stream to a port.
UDP	4	Delivers single datagrams to a port, with no delivery guarantee.
IP	3	Addresses packets and carries them across networks.
Ethernet	2	Frames with MAC addresses on one link; its cables and signals are layer 1.
Switch	2	Reads the destination MAC address.
Router	3	Reads the destination IP address.
Hub	1	Repeats every bit to every port without reading anything.
Load balancer routing by path	7	Reads the HTTP path, so it must unwrap every layer below.

The two that catch people are DNS and the load balancer. DNS feels like infrastructure, but it is a program answering questions, and its messages ride on UDP or TCP like any other application's. The load balancer's layer comes from what it reads: one that chose a server by port alone would be layer 4.

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