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No.175 Fail in Ping Test. 16-09-2013
Category : can not

Reason of the fail


Every node on Ethernet network has two addresses, one is physical(MAC) address and another is logical(IP) address.
IP networks maintain a mapping between the IP address of a device and its MAC address.
This mapping is known as the ARP cache or ARP table.
Once an IP address has been resolved to a MAC address, it will remain in the ARP cache for 2 minutes.
If a data is sent same IP address within the two minutes, the IP protocol uses the MAC address which is in the ARP cache.
Assume that the PING test is implemented continuously(under 2 minutes interval) with ezTCP configured same IP address.
After first Ping Test, next all Ping Test uses the MAC address that identifies 1st ezTCP owing to ARP cache.
Therefore, it may be failed in Ping Test except first Ping Test.


RESOLUTION


The user should delete the ARP cache before do Ping Test.
* Refer to the table below(It can be executed in Command Prompt).



Check the ARP cache arp -a (Check the ARP cache)
Delete the ARP cache arp -d [IP address] (Delete the corresponding MAC address)


Example


Assume the Ping Test of next two ezTCPs in Microsoft Windows(Windows 98, 98 SE, 2000 Pro, ME, XP Pro/Home).
--------------------------------------------------------------------------------------
ezTCP A: IP address : 10.1.0.1 MAC address : 00-30-F9-00-00-01
ezTCP B: IP address : 10.1.0.1 MAC address : 00-30-F9-00-00-02
--------------------------------------------------------------------------------------

C:>arp -a
No ARP Entries Found <------ no match information in the ARP cache

C:>ping 10.1.0.1 <------- excute Ping Test with ezTCP A
Reply from 10.1.0.1: bytes=32 time<1ms TTL=128
Reply from 10.1.0.1: bytes=32 time<1ms TTL=128
Reply from 10.1.0.1: bytes=32 time<1ms TTL=128
Reply from 10.1.0.1: bytes=32 time<1ms TTL=128

C:>arp -a
Internet Address Physical Address <------ an IP address is resolved to a MAC address
10.1.0.1 00-30-f9-00-00-01
C:>arp -d 10.1.0.1 <------ delete ARP cache

C:>arp -a <------ no match information in the ARP cache

C:>ping 10.1.0.1 <------ execute Ping Test with ezTCP B
Reply from 10.1.0.1: bytes=32 time<1ms TTL=128
Reply from 10.1.0.1: bytes=32 time<1ms TTL=128
Reply from 10.1.0.1: bytes=32 time<1ms TTL=128
Reply from 10.1.0.1: bytes=32 time<1ms TTL=128


MAC address


MAC(Media Access Control) address is a hardware address that uniquely identifies each network adapter.
MAC addresses are 12-digit hexadecimal numbers(48 bits in length).
By convention, MAC addresses are usually written in one of the following two formats:
MM:MM:MM:SS:SS:SS
MM-MM-MM-SS-SS-SS
The first half of a MAC address contains the ID numbers of the adapter manufacturer.
The second half of a MAC address represents the serial number assigned to the adapter by the manufacturer.
In the example,
00:30:f9:00:00:01
The prefix
0030f9
indicates the manufacturer is Sollae Systems.


ARP(Address Resolution Protocol)


When any device wishes to send data to another target device over Ethernet network, it must first determine the MAC address of that target given its IP address.
These IP-to-MAC address mappings are derived from an ARP cache maintained on each device.
If the given IP address does not appear in a device's cache(=ARP cache), that device cannot direct messages to that target until it obtains a new mapping. So an ARP broadcast request is sent out.
The ARP cache is then updated and data can be routed to its destination host.
If there is the match in the ARP cache, the data is immediately and directly routed to its destination host.





See also


Related Products


[ Ethernet-Module ] CSE-M53, CSE-M32, EZL-50L, EZL-50M, EZL-410
[ Ethernet-Module with RJ45 ] CSE-M73, EZL-70
[ Ethernet-Board ] EZL-60L, EZL-60M, CSE-B63
[ Ethernet-External ] CSE-H55, CSE-H53, CSE-H25, CSE-H20, CSE-H21, EZL-200L, EZL-200F, EZL-400S
[ Wireless-External ] EZL-300S, CSW-H80
[ Wireless-Module ] CSW-M83
[ PPP ] EZP-250, EZU-100, CSP-H50, EZP-EVB, EZU-EVB
[ I/O ] EZI-10, CIE-H10, CIE-M10