IPv4 Addressing and Subnetting

Updated

September 4, 2026

IPv4 Addressing and Subnetting

IPv4 addressing is the daily language of lab design: prefixes, masks, subnets, and who is on-link. Master this before multi-router labs.

Learning goals

  • Read and write CIDR prefixes (a.b.c.d/nn) fluently
  • Convert between prefix length and dotted masks
  • Design non-overlapping lab subnets and point-to-point links
  • Identify special-use and documentation ranges
  • Configure addresses on Linux and verify with ip

IPv4 structure

  • 32 bits, usually dotted decimal (four 0–255 octets)
  • Prefix length /N = number of network bits; host bits = 32 − N
  • Network address: host bits zero
  • Broadcast address (classical Ethernet/IPv4): host bits one (except special cases like /31)
Prefix Mask Classical usable hosts Common use
/32 255.255.255.255 1 Host route, loopback
/31 255.255.255.254 2 Point-to-point (RFC 3021)
/30 255.255.255.252 2 Classic P2P
/29 255.255.255.248 6 Small segment
/24 255.255.255.0 254 Common LAN
/16 255.255.0.0 many Summaries / big labs
/8 255.0.0.0 huge Rare in labs

CIDR and masks

192.168.10.0/24
network: 192.168.10.0
mask:    255.255.255.0
hosts:   192.168.10.1 – 192.168.10.254
bcast:   192.168.10.255
# Linux: add address
ip addr add 192.168.10.10/24 dev eth1
ip -br addr

Private and documentation ranges

Range Role
10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 Private (RFC 1918)—default lab space
192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24 Documentation (TEST-NET)—safe in examples
127.0.0.0/8 Loopback
169.254.0.0/16 Link-local (APIPA)
224.0.0.0/4 Multicast

Lab hygiene: pick a documented plan (10.20.0.0/16 etc.) and never invent overlapping subnets “for now.”

Subnetting for multi-tier labs

Example allocation from 10.20.0.0/16:

Use Block
Access / users A 10.20.1.0/24
Access / users B 10.20.2.0/24
Servers 10.20.10.0/24
P2P core 10.20.255.0/24 carved as /31 or /30
Loopbacks 10.20.254.0/24 as /32s

Rules:

  1. No accidental overlap
  2. Leave growth room
  3. Write the plan before deploy
  4. Align future summaries with real topology

Summarization (intro)

Advertising 10.20.0.0/22 for four /24s only works if you own the whole /22. Bad summaries blackhole traffic for space you do not have.

Action Risk
Summary too broad Blackholes
No aggregation ever Table growth later (IGP/BGP scale)

Layer 3 part continues summarization drills; foundations need the danger signal.

VLSM

Variable Length Subnet Masking: different prefix lengths in one design (/24 LAN + /31 core). Required for efficient labs—do not force everything to /24.

Host configuration checklist

  1. Address + prefix on the correct interface
  2. Up state (ip link set eth1 up)
  3. Gateway if leaving the subnet
  4. DNS only if apps need names (separate from pure L3 reachability)
ip addr add 10.20.1.10/24 dev eth1
ip link set eth1 up
ip route add default via 10.20.1.1
ip -br a
ip route

Worked subnet: /26 from a /24

Parent: 192.168.10.0/24 (256 addresses). Split into four /26s (64 addresses each):

Block Network Usable hosts Broadcast
0 192.168.10.0/26 .1–.62 .63
1 192.168.10.64/26 .65–.126 .127
2 192.168.10.128/26 .129–.190 .191
3 192.168.10.192/26 .193–.254 .255

Host bits: /26 → 6 host bits → \(2^6 = 64\) addresses → 62 usable (reserve network + broadcast) on Ethernet-style LANs. Point-to-point often uses /31 (RFC 3021) or /30.

Longest-prefix match: if a router has both 10.0.0.0/16 and 10.1.2.0/24, a packet to 10.1.2.5 uses the /24. More specific wins—plan summarization so you do not punch accidental holes.

Private vs public (RFC 1918)

Range Typical use
10.0.0.0/8 Large private / DC
172.16.0.0/12 Mid private
192.168.0.0/16 Home/lab LANs
100.64.0.0/10 CGNAT (ISP shared) — not “your” private LAN plan

Lab rule: pick one private block for the whole topology and document the plan before cabling Containerlab.

Paper drills

  1. Network, first host, last host, broadcast for 172.16.5.0/26
  2. How many /24s fit in 10.0.0.0/16?
  3. Can 10.0.1.0/24 and 10.0.1.128/25 coexist without overlap?
  4. From 10.50.0.0/16, carve three /24s and one /28 for a mgmt VLAN without overlap

Checkpoint

Design addresses for: two LANs (/24 each), one router-router link (/31), two router loopbacks (/32). No overlaps; list every interface IP.

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