Computer Networks and Security
Computer Networks and Security tests layered communication, addressing, transport behavior, and defensive cryptography. Expect subnetting, protocol selection, and security-property matching.
Core concepts
The TCP/IP model covers link, internet, transport, and application layers. IP routes packets; TCP provides reliable, ordered byte streams with a three-way handshake; UDP is connectionless with lower overhead. DNS resolves names, ARP resolves local IP addresses to link-layer addresses, and TLS protects application traffic with encryption and authentication. Symmetric encryption is fast for bulk data; asymmetric cryptography helps with key exchange and signatures; hashing provides integrity but is not encryption. Passwords should be salted and hashed with a slow algorithm.
Worked example
For 192.168.10.0/26, the mask is 255.255.255.192. The block size is 2^(32−26) = 64 addresses, so usable hosts are 64 − 2 = 62. Subnets begin at .0, .64, .128, and .192. The first usable address in the .64 subnet is 192.168.10.65, and the broadcast is 192.168.10.127.
Traps
ARP is used within the local subnet, not across the internet. TLS encrypts content but does not hide the destination IP address from network observers. Hashing cannot be reversed like encryption. Do not use ECB mode for structured data. A firewall rule that allows a port does not authenticate the user.