How do network also has a year，Speaking today, a fewAgreement with "arp" in accordance withSome confused old，So I come here to be a simple distinction。
First, that ARP (address resolution protocol)：
This is not difficult to understand，arp protocol used in the computer artTao own MAC address and IP addressin the case of，It used to get the peer IP address corresponding to theMAC address。of course，MAC table of a switch is acquired from here，This is why you generally installed network，PING test of timeThe first package always MISS、With a good network and a total first fire in the PT1 Full address。
It is this process work：
- A client broadcasts a request message corresponding to the destination IP address which is the hardware address number，While the broadcast message also comes with its own IP address and hardware address。
- After the broadcast packet received B，Remove the IP address and the hardware address of A，Add it to the address mapping table。Unicast response and returns，Response packet contains the IP address and the hardware address B。
- A response is received，Remove the IP address and the hardware address B，Add it to the address mapping table。
- After the device A and B can be a normal data transfer。
Then we say who Inverse ARP：
- A first sends a unicast message requests the corresponding IP address of the destination hardware address is how much? While the broadcast message also comes with its own IP address。
- After the broadcast packet received B，Modifying the request packet，Hardware address extracted from the FR frame header into the request packet's source hardware address field，A map of the address can be formed。Then a unicast response，IP address and hardware address included in the response packet and the IP address B of A。
- A response is received，Modify the response packet，Remove the hardware address from the header of the response packet into the source hardware address field，Then add it to the address mapping table。After the device A and B can be a normal data transfer。
Finally, to the chagrin of people talk about this RARP：
Why RARP people upset it? In fact, RARP somewhat similar in function to the DHCP protocol，Rather DHCP is to upgrade the BOOTP protocol，And in a sense it is to upgrade BOOTP or RARP protocol。The difference is that BOOTP and RARP RARP is indata link layerImplementation，The BOOTP is inApplication layerImplementation，As an upgraded version of DHCP BOOTP is implemented at the application layer。This difference is also reflected from the implementation level of the difference between RARP and BOOTP / DHCP packet encapsulation formats out，RARP directly encapsulated in an Ethernet frame，The protocol type set to 0x0800 in order to identify the packet is ARP / RARP packets，BOOTP / DHCP packets are encapsulated in UDP packets，As the data segment of UDPappeared。
Functionally say，RARP can only achieve a simple query works from the MAC address to IP address，MAC address and IP address of the RARP server isMust be statically configured in advanceOk。But it can be achieved in addition to DHCP statically assigned dynamic IP address assignment and IP address lease management, etc. relatively complex functions。(inSome time ago in this article，I wrote a simple operating principle of DHCP，You may wish to reference。）
RARP is to obtain ip address by hardware solutions provide early,But it has its own limitations,For example, RARP RARP server and client are on different network segments,The intermediate device is connected with a router, etc.,This time using the RARP becomes powerless,becauseRARP request packet can not pass the router。所以，BOOTP / DHCP emerged，It provides a good solution。 所以，It has now been replaced by substantially RARP BOOTP and DHCP，Nature gradually unknown。
RARP、BOOTP and DHCP compare：
| characteristic | RARP | BOOTP | DHCP |
| It relies on a server to assign IP addresses | yes | yes | yes |
| Message encapsulated in the UDP and IP，So they can be forwarded to a remote server | no | yes | yes |
| Clients can find their masks、Gateway、DNS server and download | no | yes | yes |
| Dynamically assigned IP address from the address pool，Without the need to know the MAC address of the client | no | no | yes |
| IP addresses allow temporary rental | no | no | yes |
| FQDN (using DNS) contains a registered client host extensions | no | no | yes |
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