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CS AO: Networking and Connectivity

CS AO · Work through your exam, one topic at a time.

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Networking and Connectivity

Networking and Connectivity

This domain tests addressing, name resolution, routing, and the toolcraft to prove where packets stop.

Core rules

  • IPv4 addressing: an address plus prefix. /24 = 255.255.255.0, 256 addresses, 254 usable. Network and broadcast addresses are not host addresses.
  • Private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16. Link-local 169.254.0.0/16 usually means DHCP failed.
  • Core services and ports: DNS 53 TCP/UDP, DHCP 67/68 UDP, HTTP 80, HTTPS 443, SSH 22, SMTP 25, NTP 123 UDP, LDAP 389, LDAPS 636, RDP 3389.
  • DNS resolution order matters: /etc/nsswitch.conf (files dns), then /etc/resolv.conf. Windows caches; use ipconfig /flushdns.
  • Routing: ip route shows the table; the default route (0.0.0.0/0) is the gateway of last resort. Use ip -br a, ss -tulpn, ping, traceroute, dig +short, tcpdump -i eth0 port 443.

Worked example

Host 10.20.5.77 cannot reach 10.20.8.10. Reasoning: both are in 10.20.0.0/16, so if the mask is /16 it is local and ARP should resolve it. If the mask is /24, 10.20.8.10 is remote and needs a gateway. Check ip -br a; if the host has 10.20.5.77/24, its local subnet is 10.20.5.0/24. Ping the gateway, then traceroute. If DNS names fail but IPs work, the problem is resolver configuration, not routing.

Traps

  • Forgetting that a VLAN or firewall can drop ICMP while TCP works.
  • Assuming ping success proves the application port is open.
  • Using netstat only; prefer ss on modern Linux.
  • Misreading a /25 or /26 boundary; calculate network and broadcast explicitly.