Structure of an IP Address

IP Addressing and Netmasks (IPv4 | IPv6)

This lesson modernizes IP addressing concepts for today’s Linux and hybrid cloud environments. We replace legacy classful rules with CIDR (Classless Inter-Domain Routing), clarify how prefix lengths (e.g., /24, /64) actually work, and align examples with current best practices (e.g., private ranges, link-local, loopback, and IPv6 conventions).

IPv4 vs IPv6: Structure | Notation

• IPv4 uses 32 bits, typically shown as dotted-decimal (e.g., 203.0.113.7). A prefix length or netmask identifies the network portion (e.g., 203.0.113.7/24 ⇢ mask 255.255.255.0).
• IPv6 uses 128 bits, written in hexadecimal, eight 16-bit blocks (e.g., 2001:db8:85a3:0:0:8a2e:370:7334). Leading zeros can be omitted and one contiguous run of zeros can be replaced by :: (e.g., 2001:db8:85a3::8a2e:370:7334).

CIDR: Network and Host (Modern Replacement for Class A/B/C)

Instead of fixed classes, CIDR uses a prefix length to define how many leading bits compose the network. The remainder belong to hosts (or subnets if you further partition). For example, 10.20.30.40/23 means the first 23 bits are the network; the last 9 bits vary per host.

# Convert between dotted masks and prefix lengths
# 255.255.255.0  → /24
# 255.255.254.0  → /23
# 255.255.252.0  → /22

# Linux quick checks
ip addr show         # list IPv4/IPv6 addresses, prefixes, MACs
ip route show        # view routes and connected prefixes

Key IPv4 Conventions (Updated)

• Network and Broadcast: In traditional IPv4 subnets, the all-zeros host portion is the network address and the all-ones host portion is the broadcast. Example: 192.0.2.0/24 ⇢ network 192.0.2.0, broadcast 192.0.2.255. (Directed broadcast is often disabled by default on routers for security.)
• Loopback: Entire 127.0.0.0/8 is loopback; 127.0.0.1 is the common host-local address.
• Unspecified: 0.0.0.0 means “this host, unspecified address” (e.g., servers bind to all interfaces). It is not “the first configured interface.”
• Private (RFC1918): 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16.
• Link-Local: 169.254.0.0/16 (APIPA/zeroconf); used when no DHCP address is obtained.
• CGNAT (RFC6598): 100.64.0.0/10 for carrier-grade NAT.
• Multicast: 224.0.0.0/4 (no broadcast in IPv6; see below).

Essential IPv6 Conventions

• Prefix lengths use / notation (e.g., /64 is standard for most LAN segments).
• No broadcast. IPv6 uses multicast instead (e.g., ff02::1 all-nodes; ff02::2 all-routers).
• Special ranges: :: (unspecified), ::1 (loopback), fe80::/10 (link-local), fc00::/7 (ULA, private-like), 2000::/3 (global-unicast).
• Address types: unicast, multicast, and anycast (delivered to the “nearest” node advertising the address).

Subnetting with CIDR (IPv4)

Subnetting divides a larger prefix into smaller ones. With 192.168.50.0/24, splitting into four equal subnets yields /26 blocks:

192.168.50.0/26    → usable hosts .1–.62,  broadcast .63
192.168.50.64/26   → usable hosts .65–.126, broadcast .127
192.168.50.128/26  → usable hosts .129–.190, broadcast .191
192.168.50.192/26  → usable hosts .193–.254, broadcast .255

IPv6 Interface IDs and SLAAC

Most IPv6 LANs use /64. Hosts typically auto-configure their Interface ID (the last 64 bits) using SLAAC. Routers advertise the network prefix with RA (Router Advertisements); hosts generate addresses like 2001:db8:1234:abcd::a1b2.

# Linux IPv6 examples
ip -6 addr add 2001:db8:1234:abcd::10/64 dev eth0
ip -6 route add default via fe80::1 dev eth0

Masks, Prefixes, and VLSM

A netmask (IPv4) or prefix length (IPv4/IPv6) specifies how many leading bits identify the network. VLSM (Variable Length Subnet Masking) lets you allocate different-sized subnets from the same aggregate, improving address efficiency.

# Quick mask ↔ prefix references
/30 = 255.255.255.252   (2 usable IPv4 hosts; point-to-point)
/29 = 255.255.255.248   (6 usable)
/28 = 255.255.255.240   (14 usable)
/24 = 255.255.255.0     (254 usable)

Common Tasks on Linux

# Show all interfaces with addresses (IPv4/IPv6)
ip addr show

# Add a temporary IPv4 address to an interface
sudo ip addr add 10.0.5.10/23 dev eth0

# Add a static route
sudo ip route add 10.42.0.0/16 via 10.0.5.1 dev eth0

# Verify connectivity and path
ping -c 4 8.8.8.8
traceroute 8.8.8.8     # or: tracepath 8.8.8.8

# Display routing tables
ip route show
ip -6 route show

Security and Legacy Notes

• Deprecate DES: Avoid DES for any cryptographic purpose; use modern ciphers (e.g., AES-GCM) and current TLS/IPsec suites.
• Treat broadcast carefully: Limit IPv4 broadcast domains; prefer unicast/multicast. Disable directed broadcasts on routers.
• Prefer IPv6 where possible: Simpler renumbering, built-in multicast, and end-to-end reachability (subject to policy and firewall).

What Changed from the Legacy Text?

• Replaced the class A/B/C table with CIDR and prefix lengths (the classful model is obsolete on modern networks).
• Corrected 0.0.0.0 to unspecified address, not “first configured interface.”
• Generalized loopback to 127.0.0.0/8, not only 127.0.0.1.
• Added RFC1918 private ranges, 169.254/16 link-local, 100.64/10 CGNAT, and IPv6 special ranges.
• Emphasized no broadcast in IPv6; use multicast.

IP Address Quiz