# DHCP Overview

* The Dynamic Host Control Protocol is used to allocate IPv4 addresses to end devices in your network dynamically.
* DHCP can also be used for a variety of different reasons within the enterprise including PXE, TFTP services, SCCM, DNS allocation, and lease times.
* DHCP can be configured within the Cisco iOS or as a service on a Windows Server.

<figure><img src="https://2373667134-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F8zF1bBBRr6t1PCyfuXHu%2Fuploads%2F2ntlXptGo8yiro7Cbmg3%2Fdhcp_image_01.png?alt=media&#x26;token=d191d555-59c6-4bff-a59c-b87ea61ea72e" alt=""><figcaption><p>Custom Trepa Tech DHCP image</p></figcaption></figure>

*

```
<figure><img src="https://2373667134-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F8zF1bBBRr6t1PCyfuXHu%2Fuploads%2FcB2r6q7tNA0UAVU9tzZ9%2Fdhcp_image_server.png?alt=media&#x26;token=b7434cfd-572b-4667-9972-2a2aa3de3635" alt=""><figcaption><p>Custom trepa tech dhcp image</p></figcaption></figure>
```

* DHCP uses a process called **"DORA"** to communicate with endpoints to delegate IP addressing or other **"DHCP Options"**

  * D - Discover (broadcast | UDP)
  * O - Offer (Unicast | UDP)
  * R - Request (Broadcast | UDP)
  * A - Acknowledgement (Unicast | UDP)

  <figure><img src="https://2373667134-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F8zF1bBBRr6t1PCyfuXHu%2Fuploads%2FgQwX7SnRjsL9hnMooarv%2FDHCP_DORA.png?alt=media&#x26;token=45b7a3a0-b228-4ea4-b287-ea438419aaf5" alt=""><figcaption><p>Custom Trepa Tech DORA image</p></figcaption></figure>
* DHCPv4 messages are encapsulated within the UDP transport protocol.
* DHCPv4 messages sent from the client use UDP source port 68 and destination port 67.
* DHCPv4 messages sent from the server to the client use UDP source port 67 and destination port 68.

## DHCP Relay

* In networking, sometimes the best design for your campus/enterprise network is to have the DHCP server be a single service running on a VM or a cluster of VMs.
* For endpoints logically segmented by VLANs and in their broadcast domains, the default-gateway interface will need to be configured with an **"IP Helper-address"** in order for endpoints to reach the DHCP server, **IF** the DHCP server is not within the same subnet/broadcast domain.
* The IP helper-address will redirect **"broadcast"** messages into **"unicast"** messages to be sent to the IP Helper-addresses destination IP address.

## VRF Support

* DHCP Pools can also be configured on Cisco iOS to support users in a Virtual Routing Forwarding Instance.
* VRFs create a separate logical routing table that is not a part of the global routing table. (much like a VM and the host OS)

## DHCP Exclusions

* In the enterprise, some endpoints you do not want to assign IP addresses dynamically.
* Any endpoint device that has a URL/DNS record associated with it should be static.
* Printers, share drives, and VTCs should also be considered for static IP addresses.
* In Cisco iOS and Windows DHCP Service, you can set **"DHCP Exclusions"** that will not allow an IP address in the subnet to be dynamically allocated.


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