Network Security Clinic Ltd

Improving network architecture, protocol design, and resource availability.

We ensure your protocols scale correctly, your users and devices always have resources they need, and critical issues like DHCP misconfigurations do not take your environment down.

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Designed for digital-first organisations

extension of your IT and security function

Compliance-aware (ISO, SOC, NCSC)

Security-first, Cyber Essentials Plus

Principles and Protocol: AWS, Azure & 3 Data Centers

This flowchart simulates DHCP Discover, Offer, Request, and Acknowledge messages across multiple clouds and DCs.

Vision in IPv4, Protocols & Subnetting for Networking

Learn IPv4 addressing and subnetting with practical Scenarios and Networking insights by Mohamed Warssame. Simplifying a complex topic step by step.

Network Protocols Overview

Planes, Protocols and their Principles

This card visualises the three main network planes (Data, Control, Management) and the protocols powering each layer.

Network Security Clinic: Mastering DHCP

Audience

IT professionals who want to master DHCP features, troubleshoot faster, and avoid IP conflicts, pool exhaustion, and downtime.

Real-World Experience

Migrating a network from North London Business Park to Barnet House with minimal DHCP issues.

What is DHCP (Dynamic Host Configuration Protocol)?

DHCP is a network protocol that automatically assigns IP addresses and other network settings (such as DNS servers and default gateways) to devices on a network.

Think of DHCP as “Google for network access.” Every device that joins a network asks DHCP for:

An IP address
DNS information
A default gateway

This process happens automatically 24/7, without user or admin intervention.

Small Networks

IT professionals can assign static IPs manually for ~20 devices.

Challenges with Manual IP Assignment in Large Networks

Tracking IP addresses for all devices
Avoiding duplicates
Renewing IPs as leases expire

DHCP Solves This By

Automatically assigning and renewing IP addresses
Preventing conflicts
Reducing administrative workload
Ensuring devices connect quickly and reliably

Is DHCP Software or Hardware?

DHCP is software, not hardware, and requires hardware to run.
Devices include: Cisco/Juniper routers & switches, Fortinet/Palo Alto firewalls, Windows/Linux servers, domain controllers.

DHCP in Cisco Networks

DHCP Tasks:

Assigns IP addresses from configured pools
Manages leases
Provides additional parameters: default gateway, DNS, domain name

Types of DHCP Clients

Directly connected LAN clients
Clients whose requests are forwarded via a DHCP relay

DHCP Relay

A DHCP relay agent forwards DHCP broadcasts from clients to a server on a different network.

Cisco Command: ip helper-address [server-ip]

Without a relay, DHCP broadcasts cannot cross routers. The relay inserts its IP address (giaddr) so the server knows the client’s subnet.

DHCP Main Functions

Provides IP address, subnet mask, default gateway, DNS server information
Uses address pools to manage allocation
Supports lease management (default 1 day)
Allows parameter inheritance: Network → Subnet → Client, with overrides at lower levels

Advanced DHCP Relay: Option 82

Adds relay agent info (Remote ID, Circuit ID)
Assign IPs based on switch port or VLAN
Differentiates clients in the same subnet
Applies policies (e.g., limit IPs per port)

DHCP Pools and Subnets

Primary subnet: main IP range

Secondary subnets: additional ranges within the same pool

Allows seamless network growth and better address utilization.

Example DHCP Pool Configuration

ip dhcp excluded-address 172.16.1.1 172.16.1.20
ip dhcp pool dhcp_1
 network 172.16.1.0 255.255.255.0
 default-router 172.16.1.1
 dns-server 8.8.8.8 1.1.1.1
 domain-name netsecclinic.com
 lease 1

DHCP FLOW – DORA Process

[Client] ---> (1) Discover ---> [Server/Relay]
[Server] ---> (2) Offer ---> [Client]
[Client] ---> (3) Request ---> [Server]
[Server] ---> (4) Ack ---> [Client]
... up to step 16 (Lease Expiration/Rebind)

Top 10 Cisco DHCP Show Commands

show ip dhcp binding
show ip dhcp pool
show running-config | include dhcp
show ip dhcp conflict
show ip dhcp server statistics
show ip dhcp snooping
show ip dhcp relay
show ip dhcp pool [pool-name]
debug ip dhcp server events
show ip dhcp lease

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