Data Communication and Computer Networks
What this domain tests
This domain covers layered models, transmission media and topologies, network devices, IPv4 addressing and subnetting, and the ports and functions of common protocols.
Layers and devices
The OSI model has seven layers: physical, data link, network, transport, session, presentation and application. TCP/IP collapses these to network access, internet, transport and application. Encapsulation turns data into segments, packets, frames and finally bits. Devices map to layers: hub at layer 1 (one broadcast domain), switch at layer 2 using MAC addresses, router at layer 3 using IP addresses, and a gateway can work up to layer 7. TCP is connection-oriented and reliable; UDP is connectionless and faster.
Addressing rules
IPv4 is 32 bits in dotted decimal. Private ranges are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16; loopback is 127.0.0.0/8. Usable hosts per subnet = 2^(host bits) minus 2, one address for the network and one for broadcast.
Worked example
Find the subnets and host range for 192.168.10.0/26. Mask = 255.255.255.192. Host bits = 6, so block size = 64 and hosts per subnet = 2^6 - 2 = 62. Subnets are .0, .64, .128 and .192. For the first subnet the usable range is 192.168.10.1 to 192.168.10.62 and the broadcast is 192.168.10.63.
Ports worth memorising
HTTP 80, HTTPS 443, FTP 20 and 21, SSH 22, Telnet 23, SMTP 25, DNS 53, DHCP 67 and 68, POP3 110, IMAP 143, SNMP 161, RDP 3389.
Traps that cost marks
- Describing a switch as a layer 3 device.
- Forgetting to subtract 2 for the network and broadcast addresses.
- Missing that DNS uses UDP 53 for queries and TCP 53 for zone transfers.
- Confusing a 48-bit MAC address with a 32-bit IPv4 address.