DNS DHCP Domain Resolution IP Assignment

DNS - Domain Name System

DNS is the "phone book" of the Internet. It translates human-readable domain names (like www.google.com) into IP addresses (like 142.250.80.46) that computers use to communicate.

DNS Hierarchy

Root DNS Servers: The top of the DNS hierarchy. There are 13 root server clusters (labeled A through M) worldwide. They know the addresses of all TLD servers.
TLD (Top-Level Domain) Servers: Manage domains like .com, .org, .net, .edu, .uk, .in. They know the addresses of authoritative name servers.
Authoritative Name Servers: Hold the actual DNS records for specific domains. Managed by domain owners or their DNS providers.
Recursive Resolver: The DNS server your ISP or organization provides. It queries other DNS servers on your behalf and caches results.

DNS Resolution Process

User types www.example.com in browser
Browser checks its local DNS cache
If not cached, OS checks its hosts file (/etc/hosts)
If not found, query goes to the Recursive Resolver (ISP's DNS)
Resolver checks its cache; if not found, queries a Root Server
Root server responds with the address of the .com TLD server
Resolver queries the .com TLD server, which responds with the authoritative name server for example.com
Resolver queries the authoritative server, which returns the IP address
Resolver caches the result and returns it to the client
Browser connects to the IP address

DNS Record Types

Record Type	Purpose	Example
A	Maps domain to IPv4 address	example.com -> 93.184.216.34
AAAA	Maps domain to IPv6 address	example.com -> 2606:2800:220:1:248:1893:25c8:1946
CNAME	Alias - maps one domain to another	www.example.com -> example.com
MX	Mail exchange - specifies mail servers	example.com -> mail.example.com (priority 10)
NS	Name server - specifies authoritative DNS servers	example.com -> ns1.example.com
PTR	Reverse DNS - maps IP to domain name	93.184.216.34 -> example.com
TXT	Text records - SPF, DKIM, domain verification	v=spf1 include:_spf.google.com ~all
SOA	Start of Authority - zone information	Primary NS, admin email, serial number, TTL
SRV	Service location records	_http._tcp.example.com -> server:port

DHCP - Dynamic Host Configuration Protocol

DHCP automatically assigns IP addresses and other network configuration (subnet mask, default gateway, DNS servers) to devices on a network. Without DHCP, every device would need to be manually configured.

DHCP DORA Process

DHCP uses a 4-step process called DORA to assign IP addresses:

Discover: Client broadcasts a DHCPDISCOVER message to find available DHCP servers. Source: 0.0.0.0, Destination: 255.255.255.255
Offer: DHCP server responds with a DHCPOFFER containing an available IP address, subnet mask, lease duration, and other options.
Request: Client broadcasts a DHCPREQUEST to accept the offered IP address (and inform other DHCP servers it's declining their offers).
Acknowledge: DHCP server sends a DHCPACK confirming the IP address assignment. The client can now use the IP address.

DHCP Lease and Static vs Dynamic IP

Feature	Static IP	Dynamic IP (DHCP)
Assignment	Manually configured	Automatically assigned by DHCP
Changes	Never changes	May change on lease renewal
Management	Complex (manual tracking)	Easy (centralized)
Use Case	Servers, printers, routers	Workstations, laptops, phones
Cost	Higher (ISP charges)	Lower

DNS packet-flow walkthrough

Client device
  -> local network interface
  -> default gateway or switch
  -> routing/security decision
  -> destination service

For DNS, explain each hop by naming the address, protocol, port, and decision made at that layer.

Separate DHCP Address Assignment from DNS Resolution

DHCP: Discover -> Offer -> Request -> Acknowledge
Lease result: address 10.20.0.42/24, gateway 10.20.0.1, DNS server 10.20.0.53
DNS: client asks 10.20.0.53 for api.example A/AAAA records
The DHCP lease supplies resolver settings; DNS then maps names to addresses.

A valid lease does not prove DNS works, and a cached DNS answer does not prove the current lease is healthy.

Before you move on