Data Networks and Internet Protocols
This domain tests OSI and TCP/IP layering, IPv4 addressing and subnetting, core protocols, and basic routing and switching behaviour. Subnetting numericals are almost certain.
Core rules
- OSI has seven layers; TCP/IP collapses them into application, transport, internet and link layers.
- IPv4 address is 32 bits; a /n prefix leaves 32 − n host bits.
- Usable hosts = 2^(host bits) − 2, subtracting network and broadcast addresses.
- Private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16; NAT translates them to public addresses.
- DHCP uses DORA: Discover, Offer, Request, Acknowledge. DNS resolves names; ARP resolves IPv4 to MAC.
- TCP is connection-oriented with acknowledgements; UDP is connectionless with lower overhead.
- Routers forward by longest-prefix match; switches forward by MAC address.
Worked example
Subnet 192.168.10.0/26. Host bits = 32 − 26 = 6, so block size = 64. Subnets are .0, .64, .128, .192. Each has 2^6 − 2 = 62 usable hosts. The third subnet is 192.168.10.128/26, usable range .129–.190, broadcast .191. Writing 64 usable hosts is the classic error because the all-zeros and all-ones host addresses are reserved.
Traps that cost marks
- Counting 2^h hosts instead of 2^h − 2.
- Confusing subnet mask 255.255.255.192 with /25.
- Assigning a broadcast address to a host.
- Mixing ARP (IP-to-MAC) with DNS (name-to-IP).
- Forgetting that NAT hides private addresses but does not replace routing.
Exam approach
Convert the prefix to host bits, compute block size, list subnets, then identify network, first host, last host and broadcast for the asked subnet.