<div>
You can have a read at this thread on both ARP and Proxy-ARP<br />
<a href="https://www.experts-exchange.com/questions/29087039/ARP-vs-Proxy-ARP.html" rel="ugc">https://www.experts-exchange.com/questions/29087039/ARP-vs-Proxy-ARP.html</a><br />
<br />
A few sentences explanation which I think is good.<br />
<a href="http://scientificsentence.net/Networking/ARP.html" rel="ugc">http://scientificsentence.net/Networking/ARP.html</a>
</div>


<div>
Not exactly the ARP / Proxy-ARP will always be handled per network segment. <br />
(It is broadcast traffic that will not travel beyond the (V)LAN.<br />
<br />
ARP is used to establist a connectionto the router, the router will establish another ARP interaction with the other party (might be an intermediate router as well).<br />
When the path is complete a packet can travel from source to destination.
</div>


<div>
noci,<br />
<br />
you explained the ARP &nbsp;
<blockquote>
ARP is used to establist a connectionto the router, the router will establish another ARP interaction with the other party (might be an intermediate router as well).
</blockquote>
<br />
That's what I thought too..<br />
What About Proxy-ARP ?
</div>


<div>
Proxy ARP was originally intended for networks with devices that can't have a default-gateway defined. &nbsp; Those devices ARP for <b>everything</b> (local and off-net). &nbsp;A router configured for proxy ARP will respond to an ARP request for an off-net device with it's own (the routers) MAC address. &nbsp;The host gets a response to the ARP request and is happy. &nbsp;The host doesn't know (or care) that the MAC address it learned is not actually the MAC of the destination device but instead is the MAC of the router.<br />
<br />
<br />
It's similar to the native VLAN concept in 802.1q trunks. &nbsp;Designed to accommodate legacy devices that didn't understand the new way of doing things. But now it's just a holdover since it's not really needed anymore.
</div>


<div>
Simply put ARP is a layer 3 to layer 2 translation protocol. &nbsp;ARP is used within a local broadcast domain to resolve IP to MAC address for l2 forwarding within the bridged infrastructure.<br />
<br />
Proxy-ARP is when a standoff l3 device is configured to respond to ARP requests for another device on a differing broadcast segment. &nbsp;Historically it is used to enable ARP responding for hidden or floating nets, non-TCP/IP translation, etc. where there is some mitigating factor where the destination host is not able to or will not respond to ARP requests for itself directly.<br />
<br />
Example:<br />
<br />
ARP:<br />
<br />
You say - who is Atlas, I have data for you<br />
I say - I am, you can send data to me at the address 123.anystreet.mi.usa<br />
<br />
ProxyARP:<br />
<br />
You say - who is Atlas, I have data for you<br />
JustInCase replies: I'm Atlas, you can send data to me at the address 456.somestreet.nj.usa<br />
JustInCase receives traffic and then forwards to me @123.anystreet.mi.usa based on the configured forwarding parameters
</div>


Software Engineer

noci

<div>
ProxyArp is ARP on behalf of another device. &nbsp; It answers &quot;by proxy&quot;. (As the name indicates.)<br />
<br />
- Note that ARP is only used my Multiple Access networks (like Ethernet, TokenRing, TokenBus etc). <br />
- For MultiDrop networks there is no need for ARP as all communication is through a central point that will poll/pass each device to send/receive info, or broadcast it to all. The central point knows each device.<br />
- For Point-t-Point network there is no need for ARP, something pushed into a cable will popup on the other end after a while. Endpoints have no need for addresses.
</div>


<div>
<blockquote>
if that's true , then ARP is within the same subnet and Proxy-ARP between 2 subnets connected by L3 device 
</blockquote>
<br />
this used to be true at some point. it was used for example to provide dhcp services on a different network.<br />
also see don johnson's post regarding setups for machines that do not do routing and use a 0 netmask.<br />
<br />
nowadays many vendors abusively use that term to qualify answering any arp request that is not targetted to the interface's main ip such as ip aliases. for example, we use multiple /27 subnets and the ISP provides a router that will redirect all ips in the subnet to whichever equipment answers to the corresponding arp query.<br />
<u>checkpoint and most linux based firewalls call the above ip aliasing an arp proxy.<br />
<br />
cisco uses this for machines that do not do routing.<br />
bsd machines <i>mostly</i> use the word for custom transparent bridging setups which is in my opinion the correct use of the term</u>
</div>


<div>
Ok Guys,<br />
<br />
*** if I have Computer1---Switch1-----co<wbr />mputer2<br />
in this case Computer1 will send an ARP Broadcast, saying &quot;My MAC address is AA.AA.AA.AA.AA.AA and My IP address is :1.1.1.1, and need to reach the IP address 2.2.2.2 of Computer 2&quot;<br />
Switch1 will register Computer1 MAC and IP address and forward the Broadcast out of all its ports except for the port the broadcast was received from<br />
Computer2 will reply to Broadcast saying &quot;&nbsp;my MAC is BB.BB.BB.BB.BB.BB and my IP address is 2.2.2.2&quot;<br />
Switch1 will register &nbsp;Computer2 MAC and IP address and forward the frame to Computer 1 as unicast<br />
<br />
This is about ARP <br />
<br />
<br />
<br />
******* if I have Computer1---Switch1-----Ro<wbr />uter1 ------Switch2------ Computer2<br />
in this case Computer1 will send an ARP Broadcast, saying &quot;My MAC address is AA.AA.AA.AA.AA.AA and My IP address is :1.1.1.1, and need to reach the IP address 2.2.2.2 of Computer 2&quot;<br />
Switch1 will register Computer1 MAC and IP address.<br />
<br />
Switch1 will register Computer1 MAC and IP address and forward the Broadcast out of all its ports except for the port the broadcast was received from<br />
Router1 will receive the Broadcast then it will send an ARP Request as Unicast to Switch2 port &nbsp;saying &quot;My MAC address is &nbsp;DD.DD.DD.DD.DD.DD , which is Router1 MAC address, and I need to get the MAC address of Computer2 that has an IP address 2.2.2.2&quot; <br />
<br />
Switch2, will send an ARP Broadcast out of all its ports except from the port it received the ARP request.<br />
<br />
Computer2 will reply to Broadcast saying &quot;&nbsp;my MAC is BB.BB.BB.BB.BB.BB and my IP address is 2.2.2.2&quot;<br />
Switch2 will register &nbsp;Computer2 MAC and IP address and forward the frame to Router1 as unicast<br />
Router1 Will use its MAC address DD.DD.DD.DD.DD.DD and IP address 2.2.2.2 of computer 2 in the frame and send it back to Switch 1, &nbsp;that will add MAC address DD.DD.DD.DD.DD.DD and IP address 2.2.2.2 &nbsp;to its Mac address table then forward the frame to Computer1 as Unicast.<br />
<br />
<br />
<br />
Is this still ARP or considered Proxy-ARP ?<br />
<br />
Thank you
</div>


<div>
On your ARP example, switch will log only the MAC to the CAM.<br />
<br />
On you proxy-ARP example.<br />
<br />
Host 1 ARP's - Who has IPw.x.y.z, tell IPa.b.c.d @MAC1.<br />
<br />
Switch logs MAC to CAM against receiving port. &nbsp;Forwards broadcast out all other ports.<br />
<br />
Router 1 sees ARP request and responds (in proxy) for Host2. &nbsp;ARP response contains MAC address of RTR interface attached to Switch1.
</div>


<div>
Your example 1 is flawed ... ARP is not used (primarily) for routing.<br />
So 1.1.1.1 -&gt; 2.2.2.2 &nbsp;will (depending on netmask) do a either an ARP (netmask &lt;6 to get 1.0.0.0/6 and 2.0.0.0/6 in the same network)<br />
Normally your gateway is looked up in the routing table. &nbsp; (fe. 1.1.1.0/24 -&gt; &nbsp; &nbsp;route add -net 2.2.2.0/24 gw 1.1.1.254 )<br />
Then the ip address for 1.1.1.254 is queried using ARP. <br />
<br />
ARP is a part of the IP protocol stack for broadcast/multicast type of networks. <br />
<br />
As atlas_shuddered already told: a switch only works with MAC addresses, <br />
A so called Layer 3 Switch actually is a switch with a router bolted onto it<br />
Switching is Layer 2, Routing is layer 3.
</div>


<div>
<blockquote>
Is this still ARP or considered Proxy-ARP ?
</blockquote>
<br />
neither<br />
<br />
in your second example, arp queries are used to discover each lan, and routing is used to join the 2 lans. this is the generic way things work on regular networks : the computers do not and cannot know each-other's mac address. they only need to know/learn the router's mac when they communicate with hosts on other lans. the arp queries are never proxied outside their respective lans.
</div>


<div>
ARP maps &nbsp;IP addresses to MAC addresses.<br />
if a router is &nbsp;netween 2 computers then the arp query/reply is handled by the Router.<br />
how do you call that ? I thought it is Arp Proxy ?
</div>


<div>
Proxy ARP is when the router responds to the request that is intended for a device on a different network.
</div>


<div>
jsk - ARP is a layer 2 protocol by definition. &nbsp;Hosts use it to talk down the stack, to transition from layer 3 to layer 2 on the way to the NIC and bits on the wire, not on the way back up the stack.<br />
<br />
Anytime a host ARP's it will always be for a local address. &nbsp;Always. &nbsp;Your local host @192.168.1.2/24 will never ARP for a remote host 10.0.0.3/22. &nbsp;Never. &nbsp;Ever. &nbsp;If your local host has to send traffic to 10.0.0.3, it will ARP for its DFG address of (in this case) 192.168.1.1 (a local address), send the traffic to it and hope for the best.<br />
<br />
proxyARP is used to manipulate this process.<br />
<br />
Using the same example above, lets assume you have another router (with the IP 192.168.1.254) included in the 192.168.1.0/24 mix. &nbsp;You have hosts that are NAT'ed behind that router. &nbsp;For the sake of argument we will say they are IP'ed 192.168.1.100, 101, 102 and 103. &nbsp;How do you get traffic to them via those NAT's. &nbsp;Your local host has it's default gateway set to 192.168.1.1. &nbsp;You could configure static mappings in your local host but chances are, if you did this for your host, you would have to do it for all the other hosts as well. &nbsp;And then you have to manage it. &nbsp;That'd suck.<br />
<br />
You proxy-ARP for these addresses on the second router. &nbsp;You build static ARP entries on the router so that it will respond to ARP requests for 192.168.1.100-103.<br />
<br />
So now, when your local host @192.168.1.2 needs to get traffic to what it sees as local addresses 192.168.1.100-103, the second router will respond to those ARP requests. &nbsp;Your host @.2 will then begin forwarding traffic to router 2 for traffic destined to 100-103.<br />
<br />
That's it, end of story. &nbsp;What other magic may happen on the router to get the traffic to the back end hosts does not matter to the example. &nbsp;What the switches do with the ARP requests and responses and what does or does not get populated into the CAM doesn't matter.<br />
<br />
The only thing that matters is that 192.168.1.254 is going to respond to any and all ARP requests for 192.168.1.100, 101, 102 and 103. &nbsp;That is proxy ARP.
</div>


<div>
<blockquote>
if a router is &nbsp;netween 2 computers then the arp query/reply is handled by the Router.<br />
how do you call that ? I thought it is Arp Proxy ? 
</blockquote>
<br />
that is not quite the way ip networks operate<br />
<br />
a host/router #A wants to talk with host #B<br />
<br />
it first checks directly connected networks : if #B is a member of one of those networks, it issues an ARP query to discover #B's mac address and communicated directly with it<br />
<br />
otherwise it checks it's routing table for the corresponding route<br />
<br />
if no matching route is found, the connection aborts immediately<br />
<br />
if a matching route is found, it issues an arp query to grab the corresponding's gateway mac address and forwards the traffic to that mac address : the header destination mac is the router's, the header destination ip is the actual destination. and the router performs the exact same operations to forward the packet either to the destination or the next router<br />
<br />
<br />
... so basically when a packet traverses multiple routers, the same level3 headers and payload is forwarded.the source and destination ips do not change. but the level2 headers are rewritten by each hop : the source mac is always the one that send/forwards the packet and the destination mac is either the final recipient's or the next_hop/gateway's
</div>


<div>
<blockquote>
in this case Computer1 will send an ARP Broadcast, saying &quot;My MAC address is AA.AA.AA.AA.AA.AA and My IP address is :1.1.1.1, and need to reach the IP address 2.2.2.2 of Computer 2&quot;
</blockquote>
<br />
this assumption is wrong on ip networks and the source of the confusion : <br />
<br />
compueter 1 will have 1.1.1.1/24 address for example which means it can only use level2 to speak with 1.1.1.*<br />
<br />
computer 1 will notice 2.2.2.2 is not on an attached network and will use it's routing table and find it's gateway is whatever address router1 has on it's own network. &nbsp;and use arp to send the packet to that address
</div>


<div>
@skullnobrains: asker did not show the netmask. &nbsp;netmask /0-/6 &nbsp;( 0.0.0.0 &nbsp;to 252.0.0.0 ) will make this in the same network.<br />
Although netmask /8 might be assumed in class-based network.
</div>


<div>
I checked this video about Proxy-ARP<br />
<br />
<a href="https://www.youtube.com/watch?v=uU1iqR_r4qc" rel="ugc">https://www.youtube.com/watch?v=uU1iqR_r4qc</a><br />
<br />
it looks like it is a feature on the router in the scenario where you cannot configure static &nbsp;or default route.<br />
in the topology presented on the video... 2 computers &nbsp;on one side are connected to the switch which in turn connected to the router, the same thing on the other side.
</div>


<div>
The switches have nothing to do with it. &nbsp;They are simply there to provide physical connectivity. <br />
<br />
In the video scenario, you need proxy arping because you have a separated /24 that overlaps with a /16 in a separate broadcast domain. &nbsp;The hosts in the /16 will never determine that the hosts in the /24 are only reachable by the gateway. They will always AND them in IP network resolution process as being part of the same broadcast domain. Because of this, the router is configured to respond to arp requests on behalf of the hosts in the /24.
</div>


jskfan

<div>
The CRUX of the movie is the scenario presented before:<br />
<br />
the bottom network (172.16.1.0/24 is a PART of the top network( 172.16.0.0/16)<br />
The Router Proxy in the Middle will do ProxyARP if one of the TopRouters needs access to the bottom network.<br />
<br />
It is also Explained as such. A very specific cornercase, and tbh. Proxy Arp is hardly used.<br />
I have used it ONCE in about 25 years to make a PLC available to other systems (PLC's might have weird IP stacks and some older ones don't understand routing).
</div>


skullnobrains

<div>
@noci: this is why i mentioned &quot;for example&quot; after adding /24 for a class C<br />
<br />
<br />
as @noci mentioned, the 2 hosts are in that case on the same network. <br />
<br />
proxy arp simply makes the level3 router join those network behaving like a level 2 device, so actually like a filtering switch<br />
<br />
this is the 3rd case i mentioned in my first post as the &quot;correct use&quot; of the term &quot;proxy arp&quot;<br />
<br />
<br />
don't do this unless you inherit a weird situation you cannot solve more elegantly
</div>


<div>
nowadays , computers send broadcast to switches and switches forwards it to the Router.<br />
I do not think we need to configure Proxy-Arp on a router <br />
<br />
Or as NOCI, mentioned above &nbsp;
<blockquote>
the bottom network (172.16.1.0/24 is a PART of the top network( 172.16.0.0/16)<br />

</blockquote>
in this case we'll have to configure Proxy-ARP on the router ??
</div>


<div>
<blockquote>
in this case we'll have to configure Proxy-ARP on the router ??
</blockquote>
<br />
That’s one way to enable communications between the two networks. &nbsp;<br />
<br />
The other (correct) way is to fix the IP addressing error that created the problem in the first place.
</div>


<div>
This question is getting wildly speculative and way off course from what was originally asked.<br />
<br />
What are ARP and pARP, what is the difference, why to use?<br />
<br />
As to building the network correctly in the first place or correcting the underlying &quot;problem&quot;, both of these may be reasonable. &nbsp;Then again, anyone that has actually been an engineer for any length of time also knows that there are any number of reasons that may lead to the situation and not all of those &quot;problems&quot; are avoidable or correctable.<br />
<br />
What ARP and pARP are has been explained to death. &nbsp;How they differ and when pARP would be used is in the same boat.<br />
<br />
pARP has a very specific purpose meant to address very specific needs. &nbsp;So when do you use it? &nbsp;When it is the best solution to the problem you are facing in your network. &nbsp;Quit overthinking the issue and getting mired down in the weeds of opinions on its particular utility, etc., etc.
</div>


<div>
<div class="content wysiwyg-content">
ARP and Proxy-ARP<br />
<br />
I have been reading about ARP and Proxy-ARP , but could not get a good grasp of it.<br />
If I understand when there is Layer3 &nbsp;between 2 hosts in different subnets, the router will take care of ARP Requests/Replies between the Hosts. <br />
in some articles they call that Proxy-ARP when the L3 device is handling the ARP Request/Reply.<br />
if that's true , then ARP is within the same subnet and Proxy-ARP between 2 subnets connected by L3 device ?<br />
<br />
Thank you
</div>
</div>


ARP and Proxy-ARP

I have been reading about ARP and Proxy-ARP , but could not get a good grasp of it.
If I understand when there is Layer3  between 2 hosts in different subnets, the router will take care of ARP Requests/Replies between the Hosts. 
in some articles they call that Proxy-ARP when the L3 device is handling the ARP Request/Reply.
if that's true , then ARP is within the same subnet and Proxy-ARP between 2 subnets connected by L3 device ?

Thank you

ARP and Proxy-ARP

A router is a networking device that forwards data packets between computer networks. Routers perform the "traffic directing" functions on the Internet. The most familiar type of routers are home and small office cable or DSL routers that simply pass data, such as web pages, email, IM, and videos between computers and the Internet. More sophisticated routers, such as enterprise routers, connect large business or ISP networks up to the powerful core routers that forward data at high speed along the optical fiber lines of the Internet backbone. Though routers are typically dedicated hardware devices, use of software-based routers has grown increasingly common.

Routers

A switch is a device that filters and forwards packets of data between LAN segments. Switches operate at the data link layer or the network layer of the Open Systems Interconnection (OSI) Reference Model and therefore support any packet protocol. LANs that use switches to join segments are called switched LANs or, in the case of Ethernet networks, switched Ethernet LANs. A hub is a connection point for devices in a network. Hubs are commonly used to connect segments of a LAN. A hub contains multiple ports; when a packet arrives at one port, it is copied to the other ports so that all segments of the LAN can see all packets.

Switches / Hubs

Networking is the process of connecting computing devices, peripherals and terminals together through a system that uses wiring, cabling or radio waves that enable their users to communicate, share information and interact over distances. Often associated are issues regarding operating systems, hardware and equipment, cloud and virtual networking, protocols, architecture, storage and management.