VLSM (Variable Length Subnet Masking) divides a parent network into variable-sized subnets according to host requirements. Algorithm: requirements are sorted from largest to smallest; for each one the smallest prefix satisfying 2^(32-cidr) − 2 ≥ hosts is selected and placed on a block boundary, contiguously.

Route summarization (supernetting) represents multiple contiguous networks with a single summarized route. Reduces routing table size and update traffic. Algorithm: identifies the common high-order bits across all start and end addresses to compute the smallest covering prefix. CIDR (RFC 4632) is the foundation of classless addressing.

Splits a parent network into equal-sized subnets. Splitting /24 → /26 produces 4 subnets (2^(26−24) = 4). FLSM (Fixed Length Subnet Masking) approach — all subnets share the same mask; simple but can waste addresses compared to VLSM.

A wildcard mask is the bitwise inverse of a subnet mask (~mask). Used in Cisco ACLs and OSPF/EIGRP network statements: a 0 bit means "this position must match exactly", a 1 bit means "this position is don't-care". For a single host use host X.X.X.X or X.X.X.X 0.0.0.0.

IPv6 addresses are 128 bits long and normalized per RFC 5952 compression rules (longest run of zero groups replaced with ::, leading zeros dropped within groups). Type detection: GUA (2000::/3), ULA (fc00::/7), link-local (fe80::/10), multicast (ff00::/8), documentation (2001:db8::/32). IPv6 has no broadcast — multicast and anycast are used instead.

DHCP Option 43 (RFC 2132 — Vendor-Specific Information) is used by APs to discover a WLC on a different subnet. Format: TLV (Type-Length-Value). For Cisco type=0xf1, for Ruckus ZD 0x06, for Ruckus SmartZone 0x03. length = number of WLC IPs × 4 (each IPv4 = 4 bytes). Multiple controller IPs can be added for HA.

Detects overlap between two subnets via integer range comparison: if A_start ≤ B_end and B_start ≤ A_end, they overlap. Relationship types: identical (same range), contains (one wholly contains the other), partial (rare for valid CIDRs — usually shows as containment). Critical for production planning and summary route safety.

Common conversions in networking: subnet mask binary analysis, MAC addresses (hex), ACL bit math, OSPF/BGP timers (decimal). Hex prefix 0x, binary 0b, octal 0o are accepted but optional. Uses BigInt, so there is no 32-bit limit — even 128-bit IPv6 values can be converted.

OSPF Cost = Reference BW / Interface BW. Minimum cost is 1. Cisco default reference is 100 Mbps — too low for modern links (1G and 10G both yield cost=1). For larger networks, raise it to 100 Gbps with auto-cost reference-bandwidth 100000. Manual override: ip ospf cost <1-65535>.

Classic EIGRP metric: (K1 × BW + K2×BW/(256-load) + K3×Delay) × (K5/(Reliability+K4)) × 256. BW = 10^7 / min_bw_kbps, Delay = sum_delay_μs / 10. Default K-values: K1=1, K3=1, others 0 — only BW and Delay are effective. When K4/K5=0 the reliability factor is not applied. On Cisco IOS the metric is shown by show ip eigrp topology.

An arbitrary IP range (S - E) generally does not fit a single CIDR; it must be split into multiple blocks. Algorithm: at each step, starting from the current S, pick the largest valid block size (bounded by S's alignment and the distance to E), then advance S += block_size. Critical for minimizing entries in firewall rules, route filters and ACLs.

Network and broadcast addresses are excluded (only usable hosts are listed). /31 and /32 are handled specially. Bash example: for ip in $(...); do ping -c1 -W1 $ip; done. With nmap a CIDR can be passed directly (nmap 192.168.1.0/24); this tool is for cases when an explicit per-target list is required.

Reverse DNS (PTR) resolves an IP back to a hostname. IPv4: octets are reversed and .in-addr.arpa is appended — 192.168.1.10 → 10.1.168.192.in-addr.arpa. IPv6: each hex nibble in reverse order, separated by dots, with .ip6.arpa appended. Used to define reverse zones in DNS server configuration. SMTP servers commonly require a valid PTR record for mail acceptance.