DHCP IP Allocation — How Dynamic IP Assignment Works

DHCP eliminates manual IP configuration by automatically assigning addresses from a pool. Every home router runs a DHCP server — your phone, laptop, and smart TV all use it to get their IP addresses.

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How DHCP works: the DORA process

DORA = Discover, Offer, Request, Acknowledge

1. DISCOVER: Client broadcasts "I need an IP address"
   Source:      0.0.0.0:68 (client has no IP yet)
   Destination: 255.255.255.255:67 (broadcast)
   Payload:     MAC address, client ID

2. OFFER: DHCP server responds with an available IP
   Source:      192.168.1.1:67 (router/server)
   Destination: 255.255.255.255:67 (still broadcast)
   Payload:     Offered IP, subnet mask, gateway, DNS, lease time

3. REQUEST: Client accepts the offer (or requests renewal)
   Source:      0.0.0.0:68 (still no IP until acknowledged)
   Destination: 255.255.255.255:67
   Payload:     "I accept the offer from server 192.168.1.1, IP 192.168.1.42"

4. ACKNOWLEDGE: Server confirms the assignment
   Source:      192.168.1.1:67
   Destination: 255.255.255.255:67 (or unicast to offered IP)
   Payload:     Confirmed IP, lease duration, all options

Result: Client configures:
  IP:        192.168.1.42
  Mask:      255.255.255.0 (/24)
  Gateway:   192.168.1.1
  DNS:       8.8.8.8, 8.8.4.4
  Lease:     24 hours

DHCP lease and renewal

Lease time: 24 hours (typical home router default)
Renewal at: T1 = 50% of lease (12 hours) — client sends unicast REQUEST
Rebinding at: T2 = 87.5% of lease (21 hours) — broadcast REQUEST if no response
Expiry: 100% of lease — client must start DORA again

Short lease (1 hour): Good for dynamic environments (conference WiFi, VMs)
Long lease (7 days): Good for stable devices (office desktops, servers)
Infinite lease: Only for static-like assignments via reservation

DHCP scope configuration

A scope is the pool of IPs a DHCP server can assign:

Example scope: 192.168.1.0/24

Total IPs:    256 (0-255)
Reserved:     
  .0          — Network address (unusable)
  .1          — Gateway (router, excluded from pool)
  .255        — Broadcast (unusable)
  .2–.49      — Static devices (printers, NAS, servers) — excluded from pool

DHCP pool:    192.168.1.50 – 192.168.1.254 (205 addresses)

Router config (dnsmasq format):
dhcp-range=192.168.1.50,192.168.1.254,24h
dhcp-option=3,192.168.1.1    # gateway
dhcp-option=6,8.8.8.8,8.8.4.4  # DNS servers

Static DHCP reservations (DHCP + static hybrid)

A reservation assigns the same IP to a specific device (by MAC address), combining static IP reliability with DHCP management:

# dnsmasq: reserve IP by MAC address
dhcp-host=00:1A:2B:3C:4D:5E,192.168.1.10,printer

# Multiple reservations:
dhcp-host=aa:bb:cc:dd:ee:ff,192.168.1.20,nas-server
dhcp-host=11:22:33:44:55:66,192.168.1.30,media-server

// ISC DHCP server (common Linux server):
// /etc/dhcp/dhcpd.conf

/*
subnet 192.168.1.0 netmask 255.255.255.0 {
  range 192.168.1.100 192.168.1.200;
  option routers 192.168.1.1;
  option domain-name-servers 8.8.8.8, 8.8.4.4;
  default-lease-time 86400;    # 24 hours
  max-lease-time 604800;       # 7 days

  # Static reservation:
  host printer {
    hardware ethernet 00:1a:2b:3c:4d:5e;
    fixed-address 192.168.1.10;
  }
}
*/

DHCP options

Beyond IP/mask/gateway/DNS, DHCP can send many options:

Option 3:   Router (default gateway)
Option 6:   DNS servers
Option 12:  Hostname
Option 15:  Domain name
Option 28:  Broadcast address
Option 42:  NTP servers
Option 43:  Vendor-specific (used by WAPs, VoIP phones)
Option 51:  Lease time
Option 66:  TFTP server (PXE boot)
Option 67:  Boot filename (PXE boot)
Option 121: Classless static routes

DHCP relay (multiple subnets)

DHCP discovers are broadcasts and don’t cross router boundaries. A DHCP relay agent forwards them to a central DHCP server:

Subnet 1: 192.168.1.0/24
Subnet 2: 192.168.2.0/24

Without relay: each subnet needs its own DHCP server
With relay: router forwards DHCP requests to 10.0.0.5 (central server)

# Linux: dhcrelay (ISC DHCP relay)
dhcrelay -i eth0 -i eth1 10.0.0.5

# The central server has scopes for both subnets:
# 192.168.1.50–254 for subnet 1
# 192.168.2.50–254 for subnet 2

DHCP vs static IP: when to use each

DHCP reservation is better than manual static: the device can still use DHCP protocol, but always gets the same IP, and the assignment is managed centrally in the DHCP server.

Related tools

• Subnet Calculator — plan your IP ranges and scopes
• IPv6 Subnetting Guide — DHCPv6 and SLAAC
• CIDR Notation Guide — understand subnet masks