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82 votes
Accepted

Why does DHCP use UDP and not TCP?

DHCP cannot use TCP as the transport protocol because TCP requires both end-points to have unique IP addresses. At the time a host is required to use DHCP, it does not have an IP address it can source ...
Mario Jost's user avatar
  • 1,729
34 votes
Accepted

Why is broadcast used at the DHCPREQUEST step?

https://www.rfc-editor.org/rfc/rfc2131#page-16 The servers receive the DHCPREQUEST broadcast from the client. Those servers not selected by the DHCPREQUEST message use the message as notification ...
boomi's user avatar
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26 votes

Why does DHCP use UDP and not TCP?

Since the source has no IP address (0.0.0.0) and the destination is everyone (255.255.255.255), it's hard to see how you would identify a particular session. But even if you could, what would be the ...
Ron Trunk's user avatar
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21 votes

Why does DHCP use UDP and not TCP?

There are multiple reasons why TCP wouldn't work for DHCP(v4.) First of all, TCP is connection-oriented. A TCP connection is defined between two particular hosts. However, when a DHCP client first ...
reirab's user avatar
  • 469
11 votes
Accepted

dhcpv6 - stateful VS stateless, what is difference between it?

A stateful address assignment involves someone keeping track of the State. Which is to say, some system exists that provides a log that certain IP addresses were assigned to certain MAC addresses. ...
Eddie's user avatar
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11 votes

Why is broadcast used at the DHCPREQUEST step?

You can potentially have multiple DHCP servers - the request is sent as broadcast to notify the other DHCP servers that have potentially sent offers that their offer isn't taken.
Zac67's user avatar
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10 votes
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Should a router use SLAAC for IPv6 address assignment?

In RFC 7084, it also states: W-1: When the router is attached to the WAN interface link, it MUST act as an IPv6 host for the purposes of stateless [RFC4862] or stateful [RFC3315] interface address ...
Benjamin Dale's user avatar
8 votes
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How IP helper address works?

Using the ip helper-address command sets up a DHCP relay. DHCP only works on the LAN to which it is connected because it uses broadcast, which cannot cross a router. To remedy that situation, DHCP ...
Ron Maupin's user avatar
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7 votes

Should a router use SLAAC for IPv6 address assignment?

RFC 4862 really describes routers within a single administrative control. RFC 7084 clarifies how customer routers should behave. This gives an ISP some freedom in how it supports IPv6 to its customers....
Ron Maupin's user avatar
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7 votes
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Importance of DHCP option 12, client host name

You wouldn't set this option on the whole scope. Rather, you'd set it for a reserved address, so that a specific host gets a defined IP address and a defined host name.
Zac67's user avatar
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7 votes

Why does DHCP use UDP and not TCP?

When the host starts up it has no knowledge of the network addresses or masks that it should use. The only way for it to communicate is via an IP broadcast (255.255.255.255) to the local network. The ...
Kevin's user avatar
  • 91
6 votes
Accepted

How to setup a lease to 1 hour

In a nutshell, yes. For the future reference of others, a snippet from this document: http://www.cisco.com/c/en/us/td/docs/ios/12_2/ip/configuration/guide/fipr_c/1cfdhcp.html#wp1001095 Configuring ...
scottm32768's user avatar
6 votes

Should a router use SLAAC for IPv6 address assignment?

Why does RFC 4862 say "Since host autoconfiguration uses information advertised by routers, routers will need to be configured by some other means." The people who wrote that document (and it's ...
Peter Green's user avatar
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6 votes

How IP helper address works?

To add to Ron's great answer: The key piece of this is that the Router sending the DHCP request on behalf of the client will unicast the request with a source address from the address space the ...
HostBits's user avatar
  • 636
6 votes

Why is broadcast used at the DHCPREQUEST step?

Because until the server sends the DHCPACK, the client still doesn’t have an IP address. It is possible for a DHCP server to respond to a request with a DHCPNACK.
YLearn's user avatar
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6 votes
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What is the difference between DHCP and NAT?

Missing from the current answers is DHCP is a protocol for configuring hosts, and runs periodically to keep their addresses and other configuration up to date. The traffic is between the host and a ...
jonathanjo's user avatar
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6 votes
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Why are DHCP ACK and REQ messages broadcast in Link Layer?

From RFC 2131 "Dynamic Host Configuration Protocol" Remembering that there can be multiple DHCP servers, the DHCPREQUEST is broadcast because: Those servers not selected by the DHCPREQUEST ...
jonathanjo's user avatar
  • 16.3k
6 votes

Does a router always have two IP addresses?

Welcome to Network Engineering! Routers route between networks, so they need interfaces in the networks they route to. Otherwise, they're like a "bridge to nowhere."
Ron Trunk's user avatar
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6 votes
Accepted

Does a router always have two IP addresses?

As others have said -- ignoring misconfigurations, bugs, errors states and so on -- a router needs to be on multiple networks and so most naturally has multiple IP addresses. But there are a couple ...
jonathanjo's user avatar
  • 16.3k
6 votes

Routing DHCP into different LANS

You need on your router a feature called DHCP relay You configure it with the IP address of your DHCP server. When the router receive a DHCP request it will send (as unicast) a request to the DHCP ...
JFL's user avatar
  • 19.7k
6 votes
Accepted

Client timeout for Cisco IOS Static DHCP Pool

It looks like your configuration for the DHCP reservation is incorrect. It seems you only have the hardware address (used for BOOTP) missing the network type, but not the client identifier (used for ...
Ron Maupin's user avatar
  • 100k
6 votes
Accepted

pfsense DHCP lease remember time settings?

You need to change the DHCP lease duration. It is a standard option in all DHCP servers. In pfsense you'll find related settings under the Services / DHCP server page, in the "other options" part: ...
JFL's user avatar
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6 votes
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What's the difference between DHCP Disabled or Relay on an access point?

DHCP Enabled - Dynamically IP address is released (allocated) for Users on Network. DHCP Disabled - Automatic IP address allocation is disabled, Need to provide manual and Static IP address ...
infra's user avatar
  • 3,138
6 votes
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Do Directly Connected Routers Need Routing Protocols For DHCP Relay To Work?

Routers need to know how to reach networks in order to send packets to those networks. DHCP relay is sending packets to other networks. Routers learn routes in three ways: Directly connected networks ...
Ron Maupin's user avatar
  • 100k
5 votes

L3 leaf-spine architecture and DHCP ip assignment

My guess would be to use a DHCP relay agent on your core switch/router. Configure this under each vlan interface and point it to your DHCP server. The DHCP discoveries will only stay on the LAN ...
SleepyMan's user avatar
  • 2,016
5 votes

How we can assume which network protocol is working in which OSI Layer?

In addition to what others have said regarding the services each layer provides, different layers typically also differ in terms of how far they are encapsulated within packets (and in what order they ...
Adam Sharpe's user avatar
5 votes
Accepted

How we can assume which network protocol is working in which OSI Layer?

The OSI model is not 100% conformed to. It is a guide line for understanding and organizing the various functions required to create a network, or an inter-network, or even the Internet. So not ...
Eddie's user avatar
  • 15.1k
5 votes

Is a DHCP offer packet a broadcast or unicast?

https://www.ietf.org/rfc/rfc2131.txt As per this RFC: Droms Standards Track [Page 24] RFC 2131 Dynamic Host Configuration Protocol March ...
Ankur Kher's user avatar
5 votes
Accepted

Assign IP address using Router and Switch (DHCP)

Switches don't get involved in layer-3 (e.g. IP). A switch is a transparent layer-2 device, so hosts connected to a switch will have no problem getting to a DHCP server on the same LAN.
Ron Maupin's user avatar
  • 100k
5 votes

How does a DHCP server differentiate between 2 requesting devices?

The DHCP server knows the layer-2 (MAC) address of the requesting host because of the CHADDR (Client hardware address) field in the DHCPDISCOVER message. The DHCPOFFER message also contains a CHADDR ...
Ron Maupin's user avatar
  • 100k

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