CCNA quiz 1: network fundamentals

Routers forward packets using IP addresses, which belong to Layer 3. Switches forward frames using MAC addresses at Layer 2.

TCP sets up a connection, numbers each segment and resends anything that is not acknowledged. UDP has none of this.

The client sends SYN, the server replies SYN-ACK and the client completes with ACK. Data can then flow.

HTTPS uses TCP 443 (and UDP 443 for HTTP/3 over QUIC). Port 80 is plain HTTP, 22 is SSH and 3389 is RDP.

A router does not forward broadcasts between its interfaces, so each interface is its own broadcast domain. VLANs on a switch also split broadcast domains.

A switch learns source MAC addresses into its MAC address table and forwards frames out of the port linked to the destination MAC.

Normal DNS queries use UDP 53. TCP 53 is used for large responses and zone transfers. UDP 67/68 is DHCP; TCP 25 is SMTP.

169.254.0.0/16 is the link-local (APIPA) range. Windows assigns itself one when no DHCP server answers.

Fibre carries light, not electricity, so it is immune to electromagnetic interference and runs much further than copper.

ARP resolves an IPv4 address to a MAC address on the local network. DNS resolves names; DHCP assigns addresses; routing protocols choose paths.

TCP and UDP work at Layer 4 and use port numbers to deliver data to the right application.

Layer 2 uses frames, Layer 3 packets and Layer 4 segments (TCP) or datagrams (UDP).

SSH uses TCP 22. Port 21 is FTP control, 23 is Telnet and UDP 69 is TFTP.

In a star topology each device has its own link to a central device, which is how modern Ethernet LANs are built.

An Ethernet MAC address is 48 bits, written as 12 hexadecimal digits. The first half identifies the manufacturer (OUI).

DHCP hands out IP settings using the Discover, Offer, Request, Acknowledge (DORA) process.

Twisted-pair Ethernet runs are limited to 100 metres (90 m permanent link plus patch leads). Cat 6 supports 10 Gbps only up to about 55 m.

A hub is a Layer 1 repeater: it copies bits out of every port without reading addresses.

ipconfig (or ipconfig /all for more detail) shows the interface addressing. tracert shows the path, nslookup tests DNS.

Ping sends ICMP echo requests and waits for echo replies. It has no port numbers.