Home | Reports | Technical Documents | Tech-Blog | One-Shot Gallery | Korea ICT News | Korea Communication Market Data | List of Contributors | Become a Contributor |    
Section 5G 4G LTE C-RAN/Fronthaul Gigabit Internet IPTV/Video Streaming IoT SDN/NFV Wi-Fi KT SK Telecom LG U+ Network Protocol Samsung   Korean Vendors
Real World Private 5G Cases   4 Deployment Models On-Premise Cases 5G Core Control Plane Sharing Cases

5G Core Sharing Cases

Private 5G Deployment   • Private 5G Frequency Allocation Status in Korea  South Korean government's regulations on private 5G and KT's strategy for entering the market
Cases in Korea   Private 5G Operators |   SK Networks Service (SI) Sejong Telecom (Wire-line Carrier) KT MOS (Affiliate of KT) • Newgens (SI) • NAVER Cloud more >>  
    Enterprise DIY |   Korea Hydro & Nuclear Power (Power Plant) Korea Electric Power Corporation (Energy) • Republic of Korea Navy more >>
CHANNELS     HFR Private 5G Solution (my5G)       my5G Solution Components       my5G Key Features        my5G Resources        my5G News          
Understanding DHCP Relay Agents
November 05, 2013 | By Netmanias (tech@netmanias.com)
Online viewer:
Comments (38)
varun chaudhary 2017-07-26 14:49:18

Awesome article. Haven't seen such a detailed and clear discription of the topic.

Netmanias 2017-07-26 16:43:31

Hi Varun, Thank you for your compliment :-)

rakesh 2017-10-30 03:48:30

super details thanks a  lot for a great article...

vikram 2017-12-08 16:43:04

I ever r seen this much clear explanatioin on dhcp relay agent. Thank you :)

Priyanshu 2017-12-11 20:30:00

This is by far the best explanation i have seen on DHCP.. thank a ton !!!

Jim 2018-03-28 10:59:25

Best article written on DHCP to the date.

Chris Changmo Yoo 2018-03-28 12:17:12

Thank you for your compliment. I'm glad you liked it. 

ranji 2018-06-05 05:57:58

excellent!!! a crystal clear explanation with enormous details... verymuch appreciated... hats off...

megs 2018-06-16 11:37:07

Wonderful article !! Thanks for the insight, it's been helpful in understanding DHCP

Muriel 2018-10-03 03:03:38

The document is pretty helpful! Thanks for sharing such an enormous details article.

Liju Philip 2018-11-09 18:59:01

Excellent document. Very useful.Thanks for sharing

Adel 2018-12-26 07:20:14

good explanation Thanks

Igor 2019-02-05 01:05:34

Very, very gooood. Big Thanks !!!

benjadavid 2019-03-30 03:40:31

omg! this is super work.

ron 2019-04-09 18:29:26

the best i ever seen


thanks !!

jtac 2019-05-14 19:53:01

you did a well job but smothing you need to edit in relp port it should be 67


victor balraj 2019-05-17 18:33:20

wonderful article...Really appriciated..Great Thanks...

Ajith Adapa 2019-07-16 01:28:10

Excellent Article.


Is it possible to know which editing tool you have used to draw such a beautiful detailed explanaions with respect to protocol ?

Chris Yoo 2019-07-24 08:23:49

We mainly use Microsoft Visio as a drawing tool.

Srini 2019-10-11 13:46:32

How does the the relay agent know the DHCP server IP and MAC?

chris 2019-10-15 11:35:32

The DHCP server IP should be configured on relay agent,

but MAC address of the DHCP server does not need to be known by the relay agent.

said 2020-05-22 11:01:57

The source port and destination port used between router and DHCP server must be 67 in both direction.

Vidhun 2020-06-18 15:42:15

Nice work, this gives a good explanation. 

Akhil 2020-06-30 03:16:37

Very informative article !! A quick note though, I think the source port from the relay agent should be 67 itself otherthan 68 whent the relay agent unicasts the packets to the Server. Correct me if i am wrong.

Akhil 2020-06-30 03:18:33

narayanpethabhilash@gmail.com 2020-07-10 13:15:10

Very useful article on DHCP relay agent 

spidy_trench 2020-07-17 16:12:56

I've been surfing internet to get packet level explaination specifially with DHCP Server located on a different network, this is the best explaination I have ever came across.


- Great Job.  I can imagine your efforts and time behind this.


Cheers from Seattle, USA

Massimiliano Sbaraglia 2021-01-24 21:52:12

It is possible to havce the same good lesson about DHCPv6 messages and if it possible understand better the transport between an client with DHCPv6 request over an IPv4 network backbone; the DHCP server is dual-stack ? Thank you too much


Rajneesh Panwar 2021-02-26 13:41:13

Very well explained and easiest way with artifacts.



san 2021-03-25 03:53:44

Wow supper ....

rbhati 2021-05-21 19:23:54

Vice nice explanation.


Can we configure any device except (router/switch) as DHCP relay agent?

If  yes, then can we configure one port as DHCP client and other port as DHCP relay agent on this device (this device neither  a router nor a switch)?  

rmanedi 2022-02-08 06:23:04

wonderful explination

Safal 2022-05-27 12:30:40

Was looking for information on how DHCP identifies it came from which subnet? Nice Explaination. Thanks for taking time and efforts.

rahul 2023-05-25 17:02:24

really nice content

Carlos 2023-11-30 16:12:36

Hi, The DHCPDISCOVER and DHCPREQUEST packets used as SIP the IP:, not the



Ali 2024-03-04 20:48:12

Hi , If between Router (DHCP Relay) and DHCP Server , there is a Firewall , does it work?

In this case , Does the client Get IP address?

Ali 2024-03-04 20:54:16

Hi , If between Router (DHCP Relay) and DHCP Server , there is a Firewall , does it work?

In this case , Does the client Get IP address?

Andreas Kreitmair 2024-06-11 00:16:25

Dear Johannes ,

I'm writing you as a fellow German bloke and wanted to ask whether you could guide mit to an authoritative source which states, what the default Source IP Address of a DHCPv4 Discover / Request from a client via Relay Agent is.

I'm asking this because refering to my tests Cisco Routers in this regard behave differently to other Network manufacturers:

Behaviour with Cisco Router:

                                      eth0:        eth1:           
DHCP-Client  -------------   Cisco Router and IPv4 Relay Agent   --------------- DHCP-Server
sip                  --->          relayed with       ---->       sip:
dip                                                         dip:

Behaviour with other Router systems:
                                 eth0:          eth1:               
DHCP-Client  -------------   Centos as Router / OpenWrt  ------------------------ DHCP-Server
sip                  --->          relayed with    ------>   sip:
dip                                                     dip:

Tested in a virtual environment (Hyper-V)
Screenshots of Wireshark traces could be supplied if needed.

Question is: Unicast from Relay to DHCP by default IP of DHCP-side interface or client-side interface?

I found amongst others the following sources stating that default should be DHCP-sided interface:
1. https://docs.commscope.com/bundle/fastiron-08030-l3guide/page/GUID-44165432-627D-48C8-87BF-8D304FEA253C.html
2. https://documentation.nokia.com/srlinux/23-3/books/interfaces/dhcp-relay.html

2. states that:
"...The DHCP relay agent relays the DHCP Discover message toward the DHCP server (unicast). If configured to do so, information is added for the circuit ID and remote ID sub-options in DHCP option 82. The relayed packet is unicast toward the DHCP servers with the following values:

    SIP = outgoing interface IP address by default. If the source-address is configured, the relayed packet instead has SIP = configured source-address..."

Reference to the relevant RFC and in case for dhcrelay daemons option for "configured source-address" would be nice.

kind regards
Andreas, Munich


Thank you for visiting Netmanias! Please leave your comment if you have a question or suggestion.
Related Contents
Netmanias Technical Documents
Netmanias Technical Documents
Netmanias Technical Documents
Netmanias Blog
Netmanias Blog

[HFR Private 5G: my5G]


Details >>







Subscribe FREE >>

Currently, 55,000+ subscribed to Netmanias.

  • You can get Netmanias Newsletter

  • You can view all netmanias' contents

  • You can download all netmanias'

    contents in pdf file







View All (181)
5G (9) 6G (1) Backbone (2) Backhaul (3) Blockchain (1) CDN (1) Carrier Ethernet (3) Charging (1) Cloud Native (1) Core (1) DHCP (4) ECM (2) EMM (16) EPS (2) Google (1) HLS (1) HTTP Adaptive Streaming (3) Handover (5) IPTV (4) Initial Attach (2) IoT (2) Korea (1) LTE (39) LTE Identification (2) LTE-A (1) MPLS (2) Mobility (2) NAT (7) Netflix (1) Network Architecture (3) Network Protocol (20) New Radio (1) OTT (1) PCRF (3) Private 5G (1) QoS (3) RCS (3) SDF (2) SDN/NFV (3) SK Telecom (2) Samsung (3) Security (5) Sk Telecom (1) Transparent Cache (1) Video Streaming (4) VoLTE (2) Wi-Fi (1) YouTube (2)
Password confirmation
Please enter your registered comment password.