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IP Addressing

Layer 2

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IP Addressing

DHCP Scope

Enter the subnet, where the gateway sits and how many addresses you reserve or exclude, and get the usable DHCP pool, the projected client demand and whether it fits (the 80 % rule). Pure subnet arithmetic, nothing is sent to any server.

Network address and prefix, e.g. 192.168.1.0/24.

The gateway takes 1 usable address and leaves the pool.

Servers, switches, APs, printers with a fixed IP outside the DHCP range.

A fixed IP handed out by DHCP to a specific MAC. Consumes pool.

Devices that will request a dynamic IP at the same time.

Headroom for growth and guests. Typical 20–30 %.

Examples
redzilla.cl — dhcp 
 
  
—
  
 
 
Usable addresses
 
—
 
—
 
 
 
DHCP pool size
 
—
 
after gateway, exclusions and reservations
 
 
 
Projected demand
 
—
 
clients + growth
 
 
 
Pool usage
 
—
 
demand / pool
 
 
  
 
DHCP pool usage
 
 
 0 / 0 0 % 
 
 
 
 
 

Rule of thumb: keep usage under 80 % for lease headroom.

 
  
 
How the subnet is split
 
 
  
ItemAddressesNote
   
 
 
 
 How it is calculated · pool and demand 
 
 1. Total addresses = 2^(32 − prefix).
            Usable = total − 2 (network and broadcast removed). Special cases:
/31 = 2 (RFC 3021) and /32 = 1.

 
 2. DHCP pool = usable − gateway (1 if applicable)
            − excluded − reservations.

 
 3. Demand = clients × (1 + growth/100),
            rounded up.

 
 4. Usage = demand / pool × 100. Verdict:
sufficient if the pool covers demand with headroom,
tight if usage exceeds 80 % and insufficient if demand
            exceeds the pool.

 
 
 
 
 
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