In a TCP/IP connection, ARP (Address Resolution Protocol) plays a decisive role. Therefore, if there are some problems when loading some sites or there is no ping of IP addresses, then try clearing the arp cache. It should be noted that this procedure is performed only from command line.
Instruction
Hi all! Today I will tell you how to see arp table on Windows. What is arp is an address recognition protocol designed to convert IP addresses to MAC addresses, often also called physical addresses. Earlier, I already told you what the cisco arp table looks like. I think that many colleagues who are just starting to get acquainted with the network infrastructure of this operating system, this information will be of great help to form the foundation. The main thing here is to understand the principle of operation and purpose, everything else is already the nuances of various vendors.
An important feature Ethernet interface is that each interface card has its own unique address. Each card manufacturer has its own pool of addresses within which it can issue cards. According to the Ethernet protocol, each interface has a 6-byte address. The address is written as six groups of hexadecimal digits, two in each (hexadecimal notation of a byte). The first three bytes are called the prefix, and they are assigned to the manufacturer. Each prefix defines 224 different combinations, which is equal to almost 17 million addresses.
In networks, there is no one-to-one correspondence between the physical address of the network interface (MAC address network card) and its IP address. Search by IP address for the corresponding Ethernet address is performed by the ARP protocol, which operates at the media access layer. The protocol supports random access memory dynamic arp table for caching received information. Open command prompt in Windows.
How to view arp table
We enter the command
Where you see the ip address on the left, and to the right you see the Physical address (mac address). This is the windows arp table.
By default, this cache lives for 300 seconds.
clearing the arp table
Done with the command
And we see that the arp table was cleared
How to add your entry to the arp table
This is done with the command
arp -s 157.55.85.212 00-aa-00-62-c6-09
Increase the lifetime of arp entries in Windows 7 by 10
Let's look at the example of Windows 8.1, how you can increase the lifetime of arp records, why it may be necessary, well, to offload the network with unnecessary traffic if you have little on the network that changes. This is done through the Windows registry.
Press Win + R and enter regedit and go to the branch
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
Here, in order to change the data storage period in the ARP cache, you need to create a DWORD Parameter, if your system has a bit depth of 32, then we create 32, if 64, then the same.
We set the name ArpCacheLife and set the value in seconds, after which you need to reboot and you will change the lifetime of the arp entry.
Here is the full help of the arp command
Displaying and modifying tables for converting IP addresses to physical,
used by the Address Resolution Protocol (ARP).
ARP -s inet_addr eth_addr
ARP -d inet_addr
ARP -a [-N if_addr] [-v]
- -a Display current ARP entries by querying the current log data. If inet_addr is given, then only the IP and physical addresses for the given computer will be displayed. If ARP uses more than one network interface, then entries for each table will be displayed.
- -g Same as -a option.
- -v Displays current ARP entries in verbose mode. All invalid entries and entries in the interface feedback will be displayed.
inet_addr Specifies an IP address. - -N if_addr Display ARP entries for the network interface specified in if_addr.
- -d Removes the node specified by inet_addr. The inet_addr parameter may contain the wildcard character * to remove all nodes.
- -s Adds a host and associates the Internet address inet_addr with the physical address eth_addr. The physical address is specified as 6 bytes (hexadecimal) separated by a dash. This link is persistent eth_addr Specifies the physical address.
- if_addr - If given, it specifies the Internet address of the interface whose address translation table is to change. If the parameter is not specified, the first available interface will be used.
RARP
Reverse ARP, the reverse ARP protocol serves to ensure that, according to the existing MAC address find out the IP address. This protocol is used in diskless machines (https://ru.wikipedia.org/wiki/Diskless_Workstation) that boot over the network. First of all, such a machine must find out its IP address and network parameters so that it can access the network, for example, to the TFTP server from which it will download boot record. The only thing this machine knows about itself is its MAC address.
In a TCP/IP connection, ARP (Address Resolution Protocol) plays a decisive role. Therefore, if there are some problems when loading some sites or there is no ping of IP addresses, then try clearing the arp cache. It is worth noting that this procedure is performed only from the command line.
Instruction
The ARP protocol is designed to determine link layer addresses (MAC addresses) from known IP addresses. This is a very important protocol, its work directly affects the performance of the network as a whole.
Purpose of the ARP protocol
For devices to communicate with each other, the transmitting device must have the IP and MAC addresses of the recipient. When one of the devices tries to communicate with another, with a known one, it needs to determine the MAC address of the recipient. incorporates a special protocol called ARP (Address Resolution Protocol - Address Translation Protocol), which allows you to automatically obtain a MAC address. On fig. The following illustrates the process for determining the MAC address associated with a known IP address.
Some devices keep special ARP tables that contain information about the MAC and IP addresses of other devices connected to the same local network. ARP tables allow you to establish a one-to-one correspondence between IP and MAC addresses. Such tables are stored in certain areas of RAM and are maintained automatically on each of the network devices (see tables below). In rare cases, you have to create ARP tables manually. Note that each computer on the network maintains its own ARP table.
Wherever data is sent by a network device, information stored in the ARP table is always used to send it (Figure below: one of the devices wants to send data to another device).
Functioning of the ARP protocol in subnets
To transfer data from one host to another, the sender must know the recipient's IP and MAC address. If it cannot obtain the physical address it is looking for from its own ARP table, a process called an ARP request is initiated, which is illustrated in Figure 2. higher.
The ARP request allows the host to determine the destination MAC address. The host creates an ARP request frame and broadcasts it to all network devices. The ARP request frame consists of two parts:
- frame header;
- ARP request messages.
In order for all devices to receive an ARP request, a broadcast MAC address is used. In the MAC addressing scheme, the broadcast address contains the hexadecimal number F in all bits and thus has the form FF-FF-FF-FF-FF-FF (This record of the MAC address is called canonical, in which parts of the address are separated by a hyphen (-) ; there is also an alternate notation where parts of the address are separated by a colon (:).). Because ARP request packets are broadcast, all network devices connected to local network, can receive such packets and pass them to protocols over high levels for further processing. If the device's IP address matches the destination IP address in the ARP broadcast request, the device replies to the sender with its MAC address. Such a message is called an ARP response.
After receiving an ARP response, the sending device of the broadcast ARP request extracts the MAC address from the sender's hardware address field and updates its ARP table. This device can now properly address packets using both MAC and IP addresses. The received information is used to encapsulate the data at the second and third layers before sending it over the network. When the data reaches its destination, the link layer checks for address matching, discards the link header that contains the MAC addresses, and passes the data to the network layer. The network layer checks for a match between its own IP address and the destination IP address contained in the third-level header. At the network layer, the IP header is discarded and the encapsulated data is passed to the next layer, the transport layer (layer 4). This process is repeated until the remaining partially decompressed data reaches the application (layer 7) where the user data part is read.
On Windows, there are some things that can only be done using the command line, due to the fact that they simply do not have an option with GUI. Some others, despite having a graphical version, can be easier to run from the command line.
Of course, I will not be able to list all these commands, but I will try to tell you about the use of some of them that I use myself.
If you have changed your address DNS servers in the connection settings (for example, due to problems opening a site), or you constantly see an error like ERR_DNS_FAIL or ERR_NAME_RESOLUTION_FAILED, then this command may come in handy. The fact is that when changing the DNS address, Windows may not use the new addresses, but continue to use those stored in the cache. Team ipconfig /flushdns will clear the name cache in Windows.
Ping and tracert - a quick way to identify network problems
If you're having trouble logging into a site, the same router settings, or other network or internet problems, the ping and tracert commands can come in handy.
If you enter the command pingyandex.en, Windows will start sending packets to the Yandex address, when they are received, the remote server will notify your computer about this. Thus, you can see if the packets are getting through, what proportion of them are lost, and at what speed the transfer is taking place. Often this command comes in handy when interacting with the router, if, for example, you cannot enter its settings.
Team tracert displays the path of transmitted packets to the destination address. With it, for example, you can determine on which node transmission delays occur.
netstat –an - display all network connections and ports
The netstat command is useful and allows you to see a wide variety of network statistics (when using various startup options). One of the more interesting use cases is running a command with the –an switch, which will list all open network connections on the computer, ports, and remote IP addresses from which connections are made.
telnet to connect to Telnet servers
By default, Windows does not have a Telnet client installed, but you can install it from Programs and Features in the Control Panel. After that, you can use the telnet command to connect to the servers without using any third party software.
These are far from all the commands of this kind that you can use in Windows and not all the options for their use, it is possible to output the result of their work to files, run not from the command line, but from the Run dialog box and others. So if effective use Windows commands you are interested and general information, presented here for novice users, is not enough, I recommend searching the Internet, it is there.
Jack Wallen explains how to clear the Address Resolution Protocol (ARP) cache and how to control it from the command line.Address Resolution Protocol - important component required to work with IP addresses in any operating system. It associates Ethernet addressing (IP addresses) with hardware addressing (MAC addresses). Without this protocol, the computer would not be able to connect to the external network, since one addressing system could not communicate with another.
An ARP cache is a collection of ARP entries (mostly dynamic) that are created by matching a hostname to an IP address and an IP address to a MAC address, which is necessary for a computer to communicate effectively with that IP.
The address map obtained during the matching process is stored on the computer in the ARP cache and remains there until it expires. This usually doesn't cause any problems, but in some cases a corrupted ARP entry can prevent you from connecting to the Internet and loading web pages. In such a situation, you can try clearing the ARP cache to resolve the issue. True, then the computer will have to create the cache again, but this happens quite quickly.
Clearing the ARP cache is done entirely from the command line, so stretch your fingers and get ready to type. After showing you how to clear the ARP cache, I'll explain how it can be controlled using certain command switches.
Clearing the cache
Step one: call the command line
Open the Start menu (Start) and type in the search bar "cmd" (without quotes), but do not click. Instead, right-click on the "cmd.exe" link and select the "Run as Administrator" option (Figure A). Confirm the operation in the User Account Control (UAC) window and you will see a command prompt window.
Figure A If the Command Prompt icon is already pinned to the Start menu, you won't need to use the search.
Step two: executing the command
To get started, run the arp -a command. It will list all the ARP entries stored on the computer (Figure B). Naturally, -a is far from the only available option. The arp command also supports the following switches:
-d- deleting an IP address (for example, arp -d 192.168.100.10);
-d -a- deletion of all entries in the ARP table;
-s- adding an entry to the ARP table (command arp -s ADDRESS MAC ADDRESS, where ADDRESS is the address to be added and MAC ADDRESS is the MAC address of the computer).
Figure B The screenshot shows the ARP cache for two different interfaces on the same computer. Click to enlarge.
To completely clear the cache, run the netsh interface ip delete arpcache command. It will delete the entire ARP cache, and the next time you connect to the network, it will start to fill up again.
How to check if the cleanup succeeded
After clearing the ARP cache, be sure to check if the operation was successful by running the arp -a command. If the cache has not been cleared, it is likely that a Windows bug is to blame, which manifests itself when the Routing and Remote Services service is enabled. Solving the problem is very easy:
1. Open the Start | Control Panel» (Start | Control Panel).
2. Select Administrative Tools under System And Security.
3. Launch the Computer Management tool.
4. Expand the "Services and Applications" section by double-clicking.
5. Highlight the Services section.
6. Find "Routing and Remote Access" in the list and double-click it.
7. Select the option "Disabled" (Disable) in the drop-down menu "Startup Type" (Startup Type).
8. Stop the service.
Now try clearing the ARP cache again. This time it should work.
Diagnostics
The ARP cache can also be used to diagnose network connectivity issues. In particular, it is worth checking if the list of invalid ARP entries with the MAC address 00-00-00-00-00-00 . If there are such entries, they must be removed using the -d switch. Let's say there is an entry in the cache like.
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