Following a Web Request Across the Network
Every step between typing a web address and seeing the page, and which part of the network does each step.
12 min
Activities: 0 of 2
One click, many protocols
An engineer at a branch office opens her laptop and types the address of the company's intranet. The page appears in under a second. In that second, at least six separate protocols did their work. Each one can fail on its own, which is why a network engineer must know the whole sequence. When a user says "the network is down", the real fault is usually one step in this chain.
In this course you will study each step in depth. Here you see the whole journey once, so the later modules have a map to hang on.
From switch-on to web page
- 1
1
Join the network
The laptop connects by cable or wireless. The switch or access point brings the link up. - 2
2
Get an address
The laptop asks the Dynamic Host Configuration Protocol (DHCP) server for an Internet Protocol (IP) address, a subnet mask, a default gateway and the name servers to use. - 3
3
Resolve the name
The laptop asks a Domain Name System (DNS) server to turn the name "intranet.example.com" into an IP address. - 4
4
Find the next hop
The server is on another network, so the laptop sends to its gateway. It uses the Address Resolution Protocol (ARP) to learn the gateway's hardware address. - 5
5
Open a connection
The laptop opens a Transmission Control Protocol (TCP) connection to port 443 on the server with a three-way handshake. - 6
6
Secure it
Transport Layer Security (TLS) checks the server's certificate and agrees encryption keys. - 7
7
Request the page
The browser sends a Hypertext Transfer Protocol (HTTP) request inside the encrypted connection, and the server replies with the page.
Which device does what
The switch in the branch forwards frames between devices on the local network. The router (often combined with a firewall) forwards packets between the branch and the head office. The firewall decides whether this connection is allowed. The servers provide the services: address assignment, name resolution and the web page itself. Every one of these is a possible point of failure, and a good diagnosis checks them in order.
Two short cases
Case 1. A user's laptop shows an address beginning 169.254. The core lesson on addressing explains that this is a self-assigned address. The failure is at step 2: the laptop asked for an address and nobody answered. Check the cable or wireless link first, then the address server.
Case 2. A user can open a site by its IP address but not by its name. Steps 1, 2, 4 and 5 work. The failure is at step 3, name resolution. Checking the server's routes or the firewall would waste time.
Put the journey in order
Order these steps as they happen when a laptop opens a secure web page on another network.
- 1Check the certificate and agree encryption
- 2Open a connection with a three-way handshake
- 3Learn the gateway's hardware address
- 4Request and receive the page
- 5Resolve the server name to an address
- 6Obtain an address, gateway and name servers
A browser shows a certificate warning for the intranet, but the page loads if the user continues. Which step is the problem at?