CompTIA Network+ (N10-008) - Subnetting, Routing Protocols & Network Architecture

Divide the CIDR prefix by 8 to count fully occupied octets (255). For the remaining prefix bits ($26-16=2$ bits in the 3rd octet), sum their binary place values ($128+64=192$). Thus, /26 equals 255.255.255.192.

Usable hosts = $2^h-2$, where $h$ is the number of host bits remaining ($32-\text{CIDR prefix}$). Subtract 2 to account for the reserved Network ID and Broadcast Address.

Subtract the interesting octet's subnet mask value from 256:
$$\text{Block Size}=256-\text{Interesting Octet Mask Value}$$
Example: For 255.255.255.224, block size is $256-224=32$. Subnets increment in steps of 32 (0, 32, 64, 96, ...).

• SSH: TCP port 22 (Encrypted command-line access)
• Telnet: TCP port 23 (Unencrypted, plain-text access)
• SFTP: TCP port 22 (Secure file transfer running over SSH)

• DNS: UDP/TCP port 53 (UDP for queries, TCP for zone transfers)
• HTTP: TCP port 80 (Unencrypted web traffic)
• HTTPS: TCP port 443 (TLS/SSL encrypted web traffic)

• DHCP: UDP 67 (Server) / UDP 68 (Client)
• SMTP: TCP 25 (Relay) / TCP 587 (Secure submission)
• POP3: TCP 110 (Unencrypted) / TCP 995 (Encrypted)
• IMAP: TCP 143 (Unencrypted) / TCP 993 (Encrypted)

• Distance-Vector (e.g., RIP): Shares entire routing tables only with directly connected neighbors at periodic intervals using hop counts or simple metrics.
• Link-State (e.g., OSPF): Advertises interface state changes to all routers in an area to build a complete topological map using Dijkstra's algorithm.

OSPF is an open standard, link-state Interior Gateway Protocol (IGP) featuring fast convergence and hierarchical structure via Area 0 (Backbone). Its metric is Cost, calculated as:
$$\text{Cost}=\frac{\text{Reference Bandwidth}}{\text{Interface Bandwidth}}$$