Level 2 · Intermediate: networking & connectivity

2.2DNS, DHCP & how the internet works

Goal: Understand DHCP (automatic addressing) and DNS (names → addresses), and know which is at fault depending on the symptom.

Estimated time: 13 min · self-paced reading · If you only have 5 min: read “The essentials in 30s” + the diagram.

The essentials in 30 seconds

DHCP automatically hands out the address, the mask, the gateway and the DNS servers: the assignment is a lease, renewed periodically.
DNS translates names (example.com) into IP addresses — the Internet's directory; without it, the network works but sites are unreachable by name.
No address (169.254) → DHCP; address OK but name KO → DNS: two diagnostic reflexes by elimination.

Running mini-case — Sofiane isolates the DNS in Algiers

Sofiane, a support technician in a distribution SME in Algiers, receives a ticket: “the delivery-tracking site no longer opens”, even though email works. Instead of suspecting everything, he tests: ping 8.8.8.8 answers, but ping suivis-livraison.dz returns “host not found”. He deduces a name-resolution problem: the DHCP placed the workstation correctly (valid address, gateway supplied, DNS server assigned), but name → address translation fails. He flushes the DNS cache, tries another DNS server, and access returns. He restates the full chain: the DHCP gives the address and the DNS server, then the DNS resolves each name before the connection.

Standalone case for this section: it returns in the lesson, the diagram, the script and the 48h challenge of section 2.2.

1. DHCP — the automatic address

DHCP (Dynamic Host Configuration Protocol) automatically hands out to the machines joining the network their IP address, their mask, their gateway and their DNS servers. It is thanks to it that a computer connects and “works” without manual configuration. The assignment is a lease (a duration): the machine “borrows” the address and renews it. If the DHCP does not answer, the machine ends up without a valid address (169.254.x.x, cf. 2.1).

2. DNS — the Internet's directory

DNS (Domain Name System) translates the readable names (e.g. example.com) into IP addresses that machines actually use. Every time a site is opened, the device queries a DNS server: “what is the address of this name?”. Without DNS, the network works but sites become unreachable by their name — a very recognisable symptom, which Sofiane isolated precisely.

3. The full chain and diagnosis by elimination

At start-up: the DHCP gives the machine an address and the address of a DNS server. Then, as soon as a name is used, the DNS resolves it to an address and the connection is established (client-server model). Two reflexes settle it: no address at all (169.254) → suspect the DHCP; address OK but the name does not open (while the IP answers) → suspect the DNS.

Common misinterpretation: “DNS and DHCP are the same thing; if a name does not open while the IP answers, the whole Internet is down; and flushing the DNS cache is useless”. No, triple error. The DHCP assigns the address, the DNS resolves names: two distinct roles. A site unreachable by its name but reachable by its IP does not signal an Internet failure, but a DNS resolution failure. And the DNS cache (a stale entry) is precisely a frequent cause to rule out first. Sofiane tests by IP to isolate the DNS.

DHCP places the machine on the network (address + DNS server); then DNS translates each name into an address before the connection.

Text version of the diagram (SEO / screen readers / translation): 1) Sofiane, a support technician in a distribution SME in Algiers, learns that the DHCP automatically hands out the address, the mask, the gateway and the DNS servers, as a renewed lease. 2) He remembers that the DNS translates names (example.com) into IP addresses — the Internet's directory — and that without it sites stay reachable by IP but no longer by their name. 3) Faced with a ticket, he tests ping 8.8.8.8 (reply OK) then ping on a name (“host not found”): the addressing is healthy, only the resolution fails → DNS lead. Lesson: diagnose by elimination — no address → DHCP; address OK but name KO → DNS.

🔎 Concrete example — 🔎 Concrete example — “The Internet half-works”

A user opens a site by its IP address, but not by its name; Sofiane tests:

> ping 8.8.8.8        ->  reply OK
> ping example.com    ->  "host not found"

Reading   : network + gateway OK (the IP answers)
Diagnosis : name -> address translation failing = DNS problem

Reading: the network and the gateway work (the IP answers); only the name → address translation fails.

Lesson: try another DNS server or flush the DNS cache — without touching the cabling or the addressing, which are healthy.

Section vocabulary

DHCP

Automatically assigns IP address, mask, gateway and DNS servers to the machines joining the network.

Simple example: Thanks to the DHCP, Sofiane's workstation configures itself on connection.

What it is not: an IP configuration typed entirely by hand.

Reflex: automatic address → DHCP.

DNS

Translates domain names (example.com) into IP addresses — the Internet's directory.

Simple example: Sofiane tests a name to check that the DNS answers.

What it is not: the service that hands out the network's IP addresses.

Reflex: name → address → DNS.

Name resolution

The operation of translating a name into an address, performed by the DNS.

Simple example: The resolution of suivis-livraison.dz fails: Sofiane suspects the DNS.

What it is not: the assignment of a DHCP lease to a new workstation.

Reflex: translate a name → resolution.

DHCP lease

Duration during which a machine “borrows” its address, then renews it.

Simple example: The workstation's lease expires and renews automatically.

What it is not: a fixed address set by hand forever.

Reflex: borrowed and renewed address → lease.

DNS server

Machine that answers name → address questions; its address is often supplied by the DHCP.

Simple example: Sofiane tries another DNS server to restore access.

What it is not: the router that performs the NAT to the outside.

Reflex: who answers names → the DNS server.

In practice — In practice — Spotting DNS and DHCP on a workstation

Display the configuration (ipconfig /all): note the assigned DNS servers, like Sofiane.
Check that the address was obtained automatically (mention DHCP enabled).
Test the resolution of a name, then compare with a test by IP address.
Conclude by elimination: no address → DHCP; name KO but IP OK → DNS.

You identify the DNS/DHCP settings and know which is at fault depending on the symptom.

Step-by-step script — deciding between DNS and DHCP like Sofiane (4 min)

1) Run ipconfig /all: note the address and the DNS servers. 2) Check that the address is normal (no 169.254) and was indeed obtained via DHCP. 3) Test ping 8.8.8.8 (an IP), then ping a site name. 4) If the IP answers and not the name: it is the DNS — flush the cache (ipconfig /flushdns) then try another DNS server. 5) If no valid address: it is the DHCP — check the link then the renewal.

Remember the order: first DHCP (the address and the DNS are assigned), then DNS (each name is translated).

Copy-paste template — my DNS / DHCP sheet

DHCP : assigns IP + mask + gateway + DNS servers (lease)
DNS  : translates a name (example.com) into an IP address
Test : ping 8.8.8.8 (IP) then ping a name
No address (169.254)  -> DHCP
Name KO but IP OK      -> DNS
DNS workaround: flush the cache, try another DNS server

The site no longer opens by its name

Sofiane tests by IP: it answers. Resolution fails → DNS lead, cache flushed.

No address assigned

The workstation falls back to 169.254: Sofiane looks at the DHCP side and renews the lease.

Some things work, others do not

Email works but not the web: Sofiane first checks name resolution.

A DNS server to change

Resolution stays temperamental: Sofiane tries another DNS server, often the first workaround.

✓ Best practices

Keep DHCP by default for the workstations.
Note the DNS servers used to react fast.
When in doubt, test by IP to isolate a DNS problem.

✗ Common pitfalls

Confusing DNS and DHCP.
Believing “no more Internet” while only the DNS is at fault.
Forgetting the DNS cache (a stale entry).

Key takeaways

DHCP = automatic address; DNS = names → addresses.
Site unreachable by name but OK by IP → DNS.
No address (169.254) → DHCP.
The web works on a request-response basis (client-server).

48h challenge — apply it tomorrow

Like Sofiane, reproduce the sequence: ping an IP (e.g. 8.8.8.8) then ping a site name. Note both results and conclude in one sentence: if the IP answers and not the name, the failure is DNS. Then specify the workaround you would apply (flush the DNS cache or change the DNS server).

Section exit checklist