Networking Fundamentals
This domain tests whether you can read a network design and predict what will actually flow: addressing, subnetting, VLAN segmentation, link aggregation, MTU and name resolution. Expect to compute host ranges and to reason about why traffic that looks correct on paper still fails.
Core rules
- A CIDR prefix defines the mask; the network and broadcast addresses are not usable host addresses.
- A gateway must sit inside the host's own subnet - a gateway outside it is unreachable without a route.
- 802.1Q tags carry VLAN IDs; a trunk carries many tagged VLANs, an access port carries one untagged VLAN.
- Both ends of a trunk must agree on the native/untagged VLAN, or untagged frames land in the wrong segment.
- Link aggregation (LACP) requires matching configuration on the switch; a static bond on one side and LACP on the other stays down.
- Jumbo frames require end-to-end MTU 9000 on every hop, including the virtual switch, physical switch and storage array.
Worked example
Host 10.20.5.70/26. A /26 borrows two bits, so the mask is 255.255.255.192 and each subnet holds 64 addresses. 64 divides into the fourth octet at 0, 64, 128, 192, so the host falls in 10.20.5.64/26: network 10.20.5.64, first usable 10.20.5.65, last usable 10.20.5.126, broadcast 10.20.5.127. A gateway of 10.20.5.129 would be invalid for this host.
Traps
Confusing /26 with /27; forgetting the two reserved addresses when counting hosts; MTU mismatch that lets small pings succeed while large transfers hang; asymmetric routing after adding a second uplink; DNS that resolves forward but has no PTR record.