Tech Study Guide

IP Addressing and Subnetting

IPv4 and IPv6 addressing, CIDR, subnets, default gateways, private address space, source selection, and route troubleshooting.

networking routing troubleshooting

IP Addressing and Subnetting

IP addressing is the shared vocabulary of routing, firewalls, Kubernetes Services, cloud VPCs, VPNs, and load balancers. Operators need to read CIDR notation quickly, identify local versus remote destinations, and understand why the selected source address matters.

Command Examples

ip addr show
ip route show
ip route get 198.51.100.10
ip -6 route show
ip -6 neigh show
getent ahosts example.com

Example output and meaning:

Command Example output What it does
ip addr show Interfaces, addresses, link state, counters, drops, and errors. Shows local interface state before blaming remote systems.
ip route show Destination, gateway, interface, and selected source address. Shows how the host will route the target flow.
ip route get 198.51.100.10 Destination, gateway, interface, and selected source address. Shows how the host will route the target flow.

CIDR and Prefixes

CIDR notation combines an address with a prefix length. The prefix is the network portion. The remaining bits identify hosts or interfaces inside that network.

Examples:

CIDR Meaning
10.0.0.0/8 Large private IPv4 block.
10.20.30.0/24 256 IPv4 addresses, commonly 254 usable host addresses.
192.0.2.10/32 One IPv4 host route.
2001:db8::/32 Documentation IPv6 prefix.
2001:db8:10::/64 Common IPv6 subnet size for one link.

Longest-prefix match wins. A /32 host route beats a /24, which beats a /16, which beats a default route.

Local Versus Routed

If the destination is inside a directly connected subnet, the host resolves a link-layer neighbor and sends directly. If not, it sends to a gateway. A wrong subnet mask can make a host ARP for a remote system that should have gone through a router.

Private, Public, and Special Ranges

Operators should recognize common special ranges:

  • RFC 1918 private IPv4: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16,
  • loopback: 127.0.0.0/8 and ::1,
  • link-local: 169.254.0.0/16 and fe80::/10,
  • documentation: 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24, and 2001:db8::/32.

Cloud and Kubernetes networks often overlap with private ranges. Overlap between a laptop VPN, VPC, Pod CIDR, Service CIDR, and on-prem network can produce confusing route and NAT failures.

IPv6 Basics

IPv6 is not just bigger IPv4. Hosts often have multiple IPv6 addresses, including link-local addresses. Neighbor Discovery replaces ARP, router advertisements can provide network configuration, and many links use /64 prefixes.

Dual-stack failures often look like intermittent application timeouts because clients may try IPv6 first, fall back to IPv4, or race both families.

Source Address Selection

The route lookup chooses not only an egress interface and next hop, but often a source address. Firewalls, replies, TLS certificates, and upstream ACLs may depend on that source address. Use ip route get to inspect the decision Linux would make.

Troubleshooting Flow

  1. Confirm the local addresses and prefix lengths.
  2. Confirm the selected route with ip route get.
  3. Confirm whether the destination should be local or routed.
  4. Check the default gateway.
  5. Check source address selection.
  6. Check for overlapping private ranges.
  7. For dual stack, test A/AAAA and IPv4/IPv6 paths separately.

Study Cards

Question

What does a CIDR prefix length describe?

Answer

How many leading address bits are the network prefix used for routing decisions.

Question

What is longest-prefix match?

Answer

The most specific matching route wins over broader matching routes.

Question

Why does source address selection matter?

Answer

Return routing, firewall policy, ACLs, and upstream expectations can depend on the chosen source address.

References