How to figure out what Ethernet address to use when you want to talk to a given Internet address? In fact, there is a separate protocol for this, called ARP ("address resolution protocol"). (Note by the way that ARP is not an IP protocol. That is, the ARP datagrams do not have IP headers.) Suppose you are on system 128.6.4.194 and you want to connect to system 128.6.4.7. Your system will first verify that 128.6.4.7 is on the same network, so it can talk directly via Ethernet. Then it will look up 128.6.4.7 in its ARP table to see if it have already known the Ethernet address. If so, it will stick on an Ethernet header, and send the packet. But suppose this system is not in the ARP table. There is no way to send the packet, because you need the Ethernet address. So it uses the ARP protocol to send an ARP request. Essentially an ARP request says "I need the Ethernet address for 128.6.4.7". Every system listens to ARP requests. When a system sees an ARP request for itself, it is required to respond. So 128.6.4.7 will see the request, and will respond with an ARP reply saying in effect "128.6.4.7 is 8:0:20:1:56:34". (Recall that Ethernet addresses are 48 bits. This is 6 octets. Ethernet addresses are conventionally shown in hex, using the punctuation shown.) Your system will save this information in its ARP table, so future packets will go directly. Most systems treat the ARP table as a cache, and clear entries in it if they have not been used in a certain period of time.

Note that ARP requests must be sent as "broadcasts". There is no way to send an ARP request directly to the right system. After all, the whole reason for sending an ARP request is that you don't know the Ethernet address. So an Ethernet address of all ones is used, i.e. ff:ff:ff:ff:ff:ff. By convention, every machine on the Ethernet is required to pay attention to packets with this as an address. So every system sees every ARP requests. They all look whether the request is for their own address. If so, they respond. If not, they could just ignore it. (Some hosts will use ARP requests to update their knowledge about other hosts on the network, even if the request isn't for them.) Note that packets whose IP address indicates broadcast (e.g. 255.255.255.255 or 128.6.4.255) are also sent with an Ethernet address.

Address Resolution Protocol (ARP) table contains the mapping of IP addresses to physical addresses. Physical addresses are sometimes referred to as Media Access Controller (MAC) addresses. The results are shown as a table with the following columns

Index

Specifies the index of the adapter.

MAC Address

Specifies the physical address.

IP Address

Specifies the IP address.

Type

Specifies the type of ARP entry. This type can have one of the following values: Static, Dynamic, Invalid, Other.

Network Info table contains network parameters for the local computer. The results are shown as a table with the following columns

Host Name

Specifies the host name for the local computer.

Domain

Specifies the domain in which the local computer is registered.

ScopeId

Specifies the DHCP scope name.

NetBios Node Type

Specifies whether the local computer uses dynamic host configuration protocol (DHCP).

Routing

Specifies whether routing is enabled on the local computer.

Proxy

Specifies whether the local computer is acting as an ARP proxy.

DNS

Specifies whether DNS is enabled on the local computer.

Current DNS Server

Specifies the current DNS server.

DNS Server List

Specifies the set of DNS servers used by the local computer.

Connections | Statistics | IP-Address | IP-Routing | Adapters/Interface | ARP