3com S7906E Installation Instruction

 

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Figure 1-2 ARP address resolution process 

 

 

If Host A is not on the same subnet with Host B, Host A first sends an ARP request to the gateway. The 

target IP address in the ARP request is the IP address of the gateway. After obtaining the MAC address 

of the gateway from an ARP reply, Host A sends the packet to the gateway. If the gateway maintains the 

ARP entry of Host B, it forwards the packet to Host B directly; if not, it broadcasts an ARP request, in 

which the target IP address is the IP address of Host B. After obtaining the MAC address of Host B, the 

gateway sends the packet to Host B. 

After obtaining the MAC address of a host, the device puts the IP-to-MAC mapping into its own ARP 

table. This mapping is used for forwarding packets with the same destination in future. 

An ARP table contains ARP entries, which fall into one of two categories: dynamic or static. 

A dynamic entry is automatically created and maintained by ARP. It can get aged, be updated by a new 

ARP packet, or be overwritten by a static ARP entry. When the aging timer expires or the interface goes 

down, the corresponding dynamic ARP entry will be removed. 

A static ARP entry is manually configured and maintained. It cannot get aged or be overwritten by a 

dynamic ARP entry.  

Using static ARP entries enhances communication security. After a static ARP entry is specified, only a 

specific MAC address is associated with the specified IP address. Attack packets cannot modify the 

IP-to-MAC mapping. Thus, communications between devices are protected. 

Static ARP entries can be classified into permanent or non-permanent. 

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A permanent static ARP entry can be directly used to forward packets. When configuring a 

permanent static ARP entry, you must configure a VLAN and an outbound interface for the entry 

besides the IP address and the MAC address. 

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A non-permanent static ARP entry has only an IP address and a MAC address configured. It 

cannot be directly used for forwarding data. If a non-permanent static ARP entry matches an IP 

packet to be forwarded, the device sends an ARP request first. If the sender IP and MAC 

addresses in the received ARP reply are the same as those in the non-permanent static ARP entry, 

the device adds the interface receiving the ARP reply to the non-permanent static ARP entry. Then 

the entry can be used for forwarding IP packets.