Wrap-up and resources

Key Takeaways

1. Subnets: masks and splitting a network: The prefix splits 32 bits between network and hosts. Block size = 256 − the mask number, and hosts = 2 to the power of host bits minus 2.
2. Transport: TCP, UDP and ports: TCP guarantees delivery and order at the cost of an extra round trip; UDP is fast with no guarantee. The port picks the program, and a connection is identified by five values.
3. Switching and VLANs: A switch learns from the source address and sends only to the destination. VLANs isolate devices on the same switch, and anything between VLANs goes through a router.
4. Routing and address translation (NAT): Every router knows only the next hop and chooses the longest matching prefix. NAT lets many devices go out on one address, telling them apart by port.
5. Dynamic routing: OSPF step by step: In OSPF every router knows the whole map and works out the shortest path from it. Cost comes from link speed, neighbours must agree on the area, timers and subnet, and when a link drops everyone recalculates within seconds.
6. What happens when you open a website?: Opening a site is a chain of six steps, each depending on the one before. When a site fails, ask: at which stop did the chain break?

Term	Definition	Part
Prefix	The number of network bits, such as /24.	1
Block size	256 − the mask number in the interesting octet.	1
Host count	2 to the power of host bits, minus 2.	1
Port	A number from 0 to 65535 that picks the program on a machine.	2
Socket	An IP address with a port, such as 203.0.113.10:443.	2
TCP	Reliable and ordered: handshake, sequence, acks, resends.	2
MAC table	Which MAC is behind which port, learned from sources.	3
Flooding	Sending a frame out every VLAN port when the destination is unknown.	3
VLAN	A virtual network and separate broadcast domain on the same switch.	3
Routing table	For each destination network: the next hop and the interface.	4
Longest prefix match	The most specific matching route wins.	4
Administrative distance	How much a route source is trusted; lower is better.	4
OSPF	A link-state routing protocol inside an organisation; administrative distance 110.	5
Hello	The neighbour discovery message to 224.0.0.5; every 10 s, Dead 40 s on Ethernet.	5
LSA	A description of a link or network that floods the area.	5
DHCP	Hands out address, mask, gateway and DNS automatically with the four DORA messages.	6
Resolver	A server that looks the name up for you and caches the answer.	6
DNS TTL	How many seconds an answer stays valid in the cache.	6

Grouped by type. Start with the official docs for precision, or an article or video for the explanation.