IPv4 Address Structure
An IPv4 address is a 32-bit number written as four decimal octets separated by periods (dotted-decimal notation). Example: 192.168.1.100. Each octet ranges from 0–255. Each octet is 8 bits (1 byte). Total IPv4 address space: 2^32 = ~4.3 billion addresses. Two parts: network portion (identifies the network) and host portion (identifies the specific device). The subnet mask determines where the network portion ends and the host portion begins. Default gateway: the router IP address that sends traffic off the local network. Every device on the same local network must have: same network address, same subnet mask, a unique host address, and the same default gateway.
IPv4 Address Classes and Private Ranges
Traditional class-based addressing (CIDR has replaced this, but classes still appear on A+): Class A: 1.0.0.0–126.255.255.255. Default mask /8 (255.0.0.0). Large networks. Class B: 128.0.0.0–191.255.255.255. Default mask /16 (255.255.0.0). Medium networks. Class C: 192.0.0.0–223.255.255.255. Default mask /24 (255.255.255.0). Small networks. Class D: 224.0.0.0–239.255.255.255. Multicast. Class E: 240.0.0.0+. Reserved/experimental. Private (RFC 1918) address ranges — not routable on the internet: 10.0.0.0–10.255.255.255 (/8 — Class A private). 172.16.0.0–172.31.255.255 (/12 — Class B private). 192.168.0.0–192.168.255.255 (/16 — Class C private). APIPA (Automatic Private IP Addressing): 169.254.0.0/16 — assigned by Windows when DHCP fails. Indicates no DHCP server reachable. Loopback: 127.0.0.1 — localhost, always refers to the local machine.
Subnet Masks and CIDR Notation
Subnet mask: 32-bit number where 1s = network portion, 0s = host portion. Written as dotted-decimal or CIDR prefix notation. Common subnet masks: /8 = 255.0.0.0 — 16,777,214 hosts per network. /16 = 255.255.0.0 — 65,534 hosts per network. /24 = 255.255.255.0 — 254 hosts per network. /25 = 255.255.255.128 — 126 hosts per network. /26 = 255.255.255.192 — 62 hosts per network. /27 = 255.255.255.224 — 30 hosts per network. /28 = 255.255.255.240 — 14 hosts per network. CIDR (Classless Inter-Domain Routing): 192.168.1.0/24 = network 192.168.1.0, subnet mask 255.255.255.0. For A+ exam: /24 (255.255.255.0) is the most commonly tested. First address = network address (not assignable). Last address = broadcast address (not assignable). Usable hosts = 2^(host bits) - 2.
DHCP (Dynamic Host Configuration Protocol)
DHCP automatically assigns IP configuration to devices. DHCP server provides: IP address, subnet mask, default gateway, DNS server address(es), lease duration. DHCP DORA process: Discover (client broadcasts looking for DHCP server), Offer (server offers IP configuration), Request (client accepts the offer), Acknowledge (server confirms and binds the lease). DHCP lease: temporary IP assignment with an expiration. Client renews at 50% of lease time, then at 87.5% if no response. Lease renewal: `ipconfig /renew` forces immediate renewal. DHCP reservation: permanently assigns a specific IP to a specific MAC address — combines dynamic convenience with static predictability. Common DHCP issues: IP conflict (two devices same IP), APIPA address (no DHCP server reached), wrong pool configuration.
DNS (Domain Name System)
DNS resolves human-readable domain names to IP addresses. Without DNS: must know IP addresses of every server. DNS hierarchy: Root servers (.) → TLD servers (.com, .org) → Authoritative nameservers (for specific domains). DNS record types for A+: A record — maps hostname to IPv4 address. AAAA record — maps hostname to IPv6 address. MX record — mail exchange server for a domain. CNAME — alias from one name to another. PTR — reverse lookup (IP to hostname). NS — nameserver records. DNS server configuration in Windows: Control Panel → Network → Adapter → Properties → IPv4 → DNS server addresses. Common DNS servers: Google (8.8.8.8, 8.8.4.4), Cloudflare (1.1.1.1), Quad9 (9.9.9.9). DNS caching: Windows caches DNS responses (TTL-based). Clear with `ipconfig /flushdns`. nslookup: command-line DNS query tool.
IPv6 Basics
IPv6 addresses the IPv4 address exhaustion problem. 128-bit addresses = 2^128 ≈ 340 undecillion addresses. Written as 8 groups of 4 hexadecimal digits separated by colons. Example: 2001:0db8:85a3:0000:0000:8a2e:0370:7334. Abbreviation rules: leading zeros in a group can be omitted. A consecutive group of all-zero groups can be replaced with :: (once per address). Full: 2001:0db8:0000:0000:0000:0000:0000:0001. Abbreviated: 2001:db8::1. Important IPv6 ranges for A+: ::1/128 — loopback (equivalent to 127.0.0.1). fe80::/10 — link-local (automatically assigned, not routed — like APIPA). fc00::/7 — unique local (private, similar to RFC 1918). 2000::/3 — global unicast (routable, public). ff00::/8 — multicast. IPv6 does not use broadcast — replaced by multicast. NDP (Neighbor Discovery Protocol) replaces ARP for address resolution in IPv6.