IP and routing
How a packet finds its way: IPv4 and IPv6 addresses, ARP on the local network, ICMP with ping and traceroute, and BGP between the Internet's operators.
- 42 cards
- 4 topics
- Free
The outline, topic by topic
Each topic is weighted the way the exam weighs it.
- IPv4 and IPv6
- ARP and the local network
- ICMP, ping and traceroute
- Routing and BGP
A few cards from the deck
What is an IPv4 address?
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A 32-bit address written as four numbers from 0 to 255, like 192.0.2.10; there are about 4.3 billion of them.
That pool ran out in the 2010s, hence NAT and IPv6.
ping answers, but the site does not load. What does that ping prove?
- That the site necessarily works
- That the cable is unplugged
- That the machine is reachable over IP, not that the web service works
- That DNS is broken
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That the machine is reachable over IP, not that the web service works
ping uses ICMP Echo: it tests the network layer, not port 443 or the application.
NAT or IPv6 to connect many devices?
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NAT shares one public address and saves IPv4, but devices can no longer be reached directly. IPv6 gives each one a public address; filtering must then come from an explicit firewall.
NAT is not a firewall by design, even though it does block unsolicited inbound connections.
Reaching a machine on the local network: after “Broadcast ARP Request: who has 192.168.1.20?”, what is the next message?
- ARP Reply: 192.168.1.20 is at this MAC address
- IP packet in a frame to the learned MAC
- Packet with TTL = 1
- ICMP Time Exceeded from router 1
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ARP Reply: 192.168.1.20 is at this MAC address
Only the machine concerned answers, directly to A. → MAC address cached