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UP Police Computer Operator study materials
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UP Police Computer Operator: Data Communication, Networking and Internet

UP Police Computer Operator · Work through your exam, one topic at a time.

All guide sections

Data Communication, Networking and Internet

Data Communication, Networking and Internet tests OSI/TCP-IP layers, devices, addressing, protocols, ports and basic subnetting. Numerical subnet questions are common.

Core rules

  • OSI seven layers: Physical, Data Link, Network, Transport, Session, Presentation, Application. TCP/IP: Network Access, Internet, Transport, Application.
  • A hub repeats to all ports at Layer 1; a switch forwards by MAC address at Layer 2; a router forwards by IP address at Layer 3.
  • IPv4 private ranges: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. Loopback is 127.0.0.0/8; APIPA is 169.254.0.0/16.
  • Ports: HTTP 80, HTTPS 443, DNS 53, DHCP 67/68, SMTP 25, POP3 110, IMAP 143, SSH 22, Telnet 23, FTP 20/21.
  • DNS resolves names to IP addresses; DHCP leases IP configuration; ARP maps an IP address to a MAC address.

Worked example

Find the subnet of 192.168.1.130/26. The mask is 255.255.255.192; block size = 256 - 192 = 64. Subnets start at .0, .64, .128 and .192. Host .130 lies in the .128 subnet; network address is .128; broadcast is .191; usable hosts are .129 to .190. Reasoning: /26 borrows 2 host bits, leaving 6 host bits, so usable hosts = 2^6 - 2 = 62.

Traps

A /24 has 254 usable addresses, not 256. A hub does not reduce collisions. DNS does not assign IP addresses; DHCP does. Do not call 192.168.1.255 a broadcast address outside its own subnet.