ARP Q&A

1. What is ARP?
   
Address Resolution Protocol (ARP) is a network protocol, which maps a network layer protocol address to a data link layer hardware address. For example, ARP is used to resolve IP address to the corresponding Ethernet address.
2. To which OSI layer does ARP belong?
   
ARP belongs to the OSI data link layer (Layer 2). ARP protocol is implemented by the network protocol driver. ARP packets are encapsulated by Ethernet headers and transmitted.
3. Which RFC specify the requirements for ARP?
   
RFC 826 specifies the ARP packet format and other details.
4. What is the use of ARP?
   
A host in an Ethernet network can communicate with another host, only if it knows the Ethernet address (MAC address) of that host. The higher level protocols like IP use a different kind of addressing scheme (like IP address) from the lower level hardware addressing scheme like MAC address. ARP is used to get the Ethernet address of a host from its IP address. ARP is extensively used by all the hosts in an Ethernet network.
5. Why a IP address needs to be mapped to a MAC address, why can't the MAC address itself is represented using the IP address?
   
The length of a MAC address is 6 bytes and the length of an IP address is 4 bytes. Obviously, the MAC address cannot be represented using the IP address. So an IP address must be mapped to the corresponding MAC address.
6. Can ARP be used in a network other than Ethernet?
   
ARP is a general protocol, which can be used in any type of broadcast network. The fields in the ARP packet specifies the type of the MAC address and the type of the protocol address. ARP is used with most IEEE 802.x LAN media. In particular, it is also used with FDDI, Token Ring, and Fast Ethernet, in precisely the same way as it is with Ethernet.
7. How does ARP resolve an IP address to an Ethernet MAC address?
   
When ARP needs to resolve a given IP address to Ethernet address, it broadcasts an ARP request packet. The ARP request packet contains the source MAC address and the source IP address and the destination IP address. Each host in the local network receives this packet. The host with the specified destination IP address, sends an ARP reply packet to the originating host with its IP address.
8. What is an ARP cache?
   
ARP maintains the mapping between IP address and MAC address in a table in memory called ARP cache. The entries in this table are dynamically added and removed.
9. When is an ARP request packet generated?
   
The following steps results in the generation of an ARP request packet:
1. The IP module sends a packet, destined for another host in the network, to the ARP module.
2. The ARP module looks up the ARP table (cache) to resolve the IP address.
3. If the supplied IP address is present in the ARP cache, it is resolved into its Ethernet address.
4. If the ARP module is not able to find an entry for this IP address in the ARP cache, then it sends an ARP request packet to the Ethernet driver, to resolve the IP address to the Ethernet address.
5. After the IP address is resolved by the ARP module, the packet is sent to the Ethernet driver for transmission.
10. What happens when a host receives an ARP request packet?
    
The ARP request is received and processed by all the hosts in the network, since it is a broadcast packet. The following steps are carried out when a ARP request packet is received by a host:
1. If the IP address to be resolved is for this host, then the ARP module sends an ARP reply packet with its Ethernet MAC address.
2. If the IP address to be resolved is for this host, then the ARP module updates its ARP cache with the source Ethernet MAC address to source IP address mapping present in the ARP request packet. If the entry is already present in the cache, it is overwritten. If it is not present, it is added.
3. If the IP address to be resolved is not for this host, then the ARP module discards the ARP request packet.
11. Will a host update its ARP cache upon receiving any ARP request?
    
A host will update its ARP cache, only if the ARP request is for its IP address. Otherwise, it will discard the ARP request.
12. What is the disadvantage if a host updates its ARP cache upon receiving any ARP request?
    
The host will exhaust the ARP cache with a lot of unused ARP entries, if it updates the ARP cache for any ARP request.
13. What happens when a host receives an ARP reply packet?
    
The ARP reply packet is received only by the host, which transmitted the ARP request packet. The ARP module adds the Ethernet hardware address to IP address mapping present in the ARP reply packet to the ARP cache.
14. Is there a separate packet format for ARP request and ARP reply?
    
No. Both the ARP request and ARP reply packets use the same format.
15. Which MAC address is an ARP request directed to?
    
All ARP request packets are transmitted with the Ethernet broadcast address, so that all hosts in the network will receive the request.
16. To which MAC address is an ARP reply packet directed to?
    
ARP reply packet is directed to the host, which transmitted the ARP request packet.
17. If a host is not able to get the MAC address of a host, how it knows about its IP address?
    
A host will either use a static file like /etc/hosts or DNS protocol to get the IP address of another host.
18. What will happen if an ARP reply is not received for an ARP request?
    
If an ARP reply is not received, then that IP address cannot be resolved to an Ethernet address. Without a Ethernet address, the packets cannot be transmitted.
19. When is an entry added to the ARP cache?
    
A new entry is added to the ARP cache when an IP address is successfully mapped to a MAC address. Usually, entries are added dynamically to the ARP cache. Static entries can also be added.
20. What will happen if a new ARP request packet is received, but the MAC address to IP address is already present in the ARP cache?
    
If a ARP request packet is received and the mapping already exists in the ARP cache, it will be overwritten with the values present in the request.
21. When is an entry removed from an ARP cache?
    
An entry in an ARP cache is removed after a pre-determined timeout period (e.g. 20 minutes).
22. What is the format of an ARP packet?
    
The various fields of a ARP request/reply packet and their length are shown below:

    +--------+
   Hardware 2 bytes
   MAC    
   Address
   Type   
   +--------+
   Protocol 2 bytes
   Address
   Type   
   +--------+
   Hardware 1 byte
   MAC    
   Address
   Size   
   +--------+
   Protocol 1 byte
   Address
   Size   
   +--------+
   Op       2 bytes
   +--------+
   Sender   6 bytes (depends on the above size field)
   MAC    
   Address
   +--------+
   Sender   4 bytes (depends on the above size field)
   IP     
   Address
   +--------+
   Target   6 bytes (depends on the above size field)
   MAC    
   Address
   +--------+
   Target   4 bytes (depends on the above size field)
   IP     
   Address
   +--------+

The fields are further explained below:

    +---------+-------------------------------------------------------+
   Ethernet For a ARP request, source MAC address is the MAC      
   Header   address of the host sending the ARP request,          
            destination MAC address is the Ethernet broadcast     
            address (FF:FF:FF:FF:FF:FF), frame type field is 0x806.
            For ARP reply, source MAC address is the MAC address of
            the host replying to the ARP request, destination MAC 
            address is the MAC address of the host that sent the  
            ARP request, and the frame type field is 0x806.       
   +---------+-------------------------------------------------------+
   Hardware Type of the hardware MAC address which is being mapped.
   Address  For Ethernet the value of this field is 1.            
   Type                                                           
   +---------+-------------------------------------------------------+
   Protocol Type of the protocol address to which the MAC address 
   Address  is mapped.  For IP address the value of this field is 
   Type     0x800.                                                
   +---------+-------------------------------------------------------+
   Hardware Size of the hardware MAC address.  For Ethernet, the  
   Address  value of this field is 6.                             
   Size                                                           
   +---------+-------------------------------------------------------+
   Protocol Size of the protocol address.  For IP, the value of   
   Address  this field is 4.                                      
   Size                                                           
   +---------+-------------------------------------------------------+
   OperationType of operation being performed.  The value of this 
            field can be 1 (ARP request), 2 (ARP reply)           
   +---------+-------------------------------------------------------+
   Source   The hardware MAC address of the host sending the ARP  
   MAC      request or reply.  This is same as the source MAC     
   address  address present in the Ethernet header.               
   +---------+-------------------------------------------------------+
   Source   The IP address of the host sending the ARP request or 
   IP       reply.                                                
   address                                                        
   +---------+-------------------------------------------------------+
   Target   The hardware MAC address of the host receiving the ARP
   MAC      request or reply.  This is same as the destination MAC
   address  address present in the Ethernet header.               
   +---------+-------------------------------------------------------+
   Target   The IP address of the host receiving the ARP request  
   IP       or reply.                                             
   address                                                        
   +---------+-------------------------------------------------------+

23. What is the size of an ARP request and reply packet?
    
The size of an ARP request or reply packet is 28 bytes.
24. How to differentiate between a ARP request packet and a ARP reply packet, as the Ethernet type field is same on both the packets?
    
An ARP request packet can be differentiated from an ARP reply packet using the 'operation' field in the ARP packet. For a ARP request it is 1 and for an ARP reply it is 2.
25. Why is the hardware MAC address present in both the Ethernet header and the ARP packet (request and reply)?
    
The Ethernet header is processed by the data link driver and removed from the packet. When the ARP layer gets the packet, it needs to know the hardware and protocol addresses in order to update the table. That is why the hardware MAC address is present in both the Ethernet header and the ARP packet.
26. What is proxy ARP?
    
Proxy ARP is the process in which one system responds to the ARP request for another system. For example, host A sends an ARP request to resolve the IP address of host B. Instead of Host B, Host C responds to this ARP request.
27. What is the use of proxy ARP?
    
When routers receive ARP requests from one network for hosts on the network, they will respond with a ARP reply packet with their MAC address. For example, let us say host A is in one network, host B is in another network and router C connects these two networks. When host A sends an ARP request to resolve the IP address of host B, the router C receives this packet. The router C sends an ARP reply with its MAC address. So host A will send all the packets destined for host B to the router C. Router C will then forward those packets to host B. Proxy ARP is also used if a host in a network is not able to understand subnet addressing. For example, if host A and host B are actually in two different subnets, but host A cannot understand subnet addressing. So host A assumes that host B is present in the same network. In this case a router, host C, can use proxy ARP to route packets between host A and host B.
28. What is gratuitous ARP?
    
When a host sends an ARP request to resolve its own IP address, it is called gratuitous ARP. In the ARP request packet, the source IP address and destination IP address are filled with the same source IP address itself. The destination MAC address is the Ethernet broadcast address (FF:FF:FF:FF:FF:FF).
29. What is the use of gratuitous ARP?
    
Gratuitous ARP is used for the following:
1. In a properly configured network, there will not be an ARP reply for a gratuitous ARP request. But if another host in the network is also configured with the same IP address as the source host, then the source host will get an ARP reply. In this way, a host can determine whether another host is also configured with its IP address.
2. When the network interface card in a system is changed, the MAC address to its IP address mapping is changed. In this case, when the host is rebooted, it will send an ARP request packet for its own IP address. As this is a broadcast packet, all the hosts in the network will receive and process this packet. They will update their old mapping in the ARP cache with this new mapping.