Configuring Cisco IOS DHCP Server

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Lesson 92Interactive lesson

Configuring Cisco IOS DHCP Server

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Lesson format18 sections3 code blocks1 practice itemUpdated Oct 2, 2026

bash

! Step 1: Exclude addresses the router should not lease
R1(config)# ip dhcp excluded-address 10.10.10.1 10.10.10.10
R1(config)# ip dhcp excluded-address 10.10.10.200 10.10.10.254

! Step 2: Create the pool
R1(config)# ip dhcp pool USERS
R1(dhcp-config)# network 10.10.10.0 255.255.255.0
R1(dhcp-config)# default-router 10.10.10.1
R1(dhcp-config)# dns-server 8.8.8.8 8.8.4.4
R1(dhcp-config)# domain-name corp.example.com
R1(dhcp-config)# lease 7
R1(dhcp-config)# exit

! Verify
R1# show ip dhcp pool
R1# show ip dhcp binding

The pool defines what the router can lease. `excluded-address` removes static IPs (routers, servers, printers) from the pool. `network` sets the pool's range. `default-router` is the gateway clients should use. `dns-server` and `domain-name` are passed as DHCP options. `lease 7` sets a seven-day lease.

#Understanding Excluded Addresses#link

▪Reserve for infrastructure: routers, switches, printers, servers
▪Exclude the network and broadcast addresses (unusable anyway but avoids confusion)
▪Common practice: exclude the first 10 and last 50 addresses in each subnet
▪Always document which IPs are excluded and why

callout

If you don't exclude the gateway address, the router may lease it to a client, causing an IP conflict the moment that client tries to use it. Always exclude at least the gateway IP. Excluding the entire top and bottom of a subnet is a habit worth building.

#The Full Pool Configuration#link

Command	Purpose
`ip dhcp excluded-address <start> [end]`	Prevents leasing these addresses
`ip dhcp pool <name>`	Creates a new DHCP pool
`network <net> <mask>`	Defines the subnet to lease from
`default-router <ip>`	Gateway(s) handed to clients
`dns-server <ip1> [ip2]`	DNS servers handed to clients
`domain-name <name>`	Domain suffix for clients
`lease <days> [hours] [minutes]`	Lease duration
`lease infinite`	Never expires (rare, avoid)

#Verifying DHCP#link

See active leases

root@vulnarex:~#show ip dhcp binding

Each binding shows the IP, the client identifier (usually a MAC-derived value), and the lease expiration. If a client's MAC-based ID appears twice with different IPs, they may have changed NIC or the client identifier changed — worth investigating.

#DHCP Statistics and Conflict Detection#link

Check pool utilization

root@vulnarex:~#show ip dhcp pool USERS

`Utilization mark` shows how full the pool is. `Leased addresses` tells you how many clients currently hold leases. If leased approaches total, the pool is nearly exhausted — either add more addresses or reduce the lease time so addresses recycle faster.

#Check Your Understanding#link

Configure a DHCP pool for the 10.10.20.0/24 subnet on a lab router. Exclude the first 10 and last 10 addresses. Set the gateway to 10.10.20.1, DNS to 8.8.8.8, lease to 1 day. Verify with `show ip dhcp pool` and test with a client.

★ 20 XPmedium

challenge BLOCK (★ 20 XP)

Clients on a subnet can't get an IP. `show ip dhcp pool` shows zero leased addresses and the pool is active. What should you check first?

Select your proof vectors above

#Common Mistakes#link

▪Forgetting to exclude the gateway IP — causes conflicts
▪Pool `network` doesn't match the interface IP/subnet
▪Missing `default-router` — clients get an IP but no gateway
▪Not setting DNS — clients use fallback DNS and fail name resolution
▪Lease time too long — pool exhausts during peak demand

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Understanding Excluded Addresses The Full Pool Configuration Verifying DHCP DHCP Statistics and Conflict Detection Check Your Understanding Common Mistakes

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October 2, 2026

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MCP Server Config (mcp.json)

{
  "mcpServers": {
    "vulnarex": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-fetch"],
      "env": { "MCP_FETCH_URL": "https://vulnarex.com" }
    }
  }
}

MCP Server Card/.well-known/mcp.json A2A Agent Card/.well-known/agent-card.json API Catalog/.well-known/api-catalog robots.txt/robots.txt

Laboratory sanity code

Isolate active probes on matched virtual networks and keep execution streams sandboxed.

! Step 1: Exclude addresses the router should not lease

R1(config)# ip dhcp excluded-address 10.10.10.1 10.10.10.10

R1(config)# ip dhcp excluded-address 10.10.10.200 10.10.10.254

! Step 2: Create the pool

R1(config)# ip dhcp pool USERS

R1(dhcp-config)# network 10.10.10.0 255.255.255.0

R1(dhcp-config)# default-router 10.10.10.1

R1(dhcp-config)# dns-server 8.8.8.8 8.8.4.4

R1(dhcp-config)# domain-name corp.example.com

R1(dhcp-config)# lease 7

R1(dhcp-config)# exit

! Verify

R1# show ip dhcp pool

R1# show ip dhcp binding

Command

Purpose

`ip dhcp excluded-address <start> [end]`

Prevents leasing these addresses

`ip dhcp pool <name>`

Creates a new DHCP pool

`network <net> <mask>`

Defines the subnet to lease from

`default-router <ip>`

Gateway(s) handed to clients

`dns-server <ip1> [ip2]`

DNS servers handed to clients

`domain-name <name>`

Domain suffix for clients

`lease <days> [hours] [minutes]`

Lease duration

`lease infinite`

Never expires (rare, avoid)