While testing the correct MTU size of your current network, sometimes you may receive the error message "Packet needs to be fragmented but DF set". In this tutorial let us check the reason and solution for this error message. If the MTU size you have set on the ping test is too high, you will get this message. Let us check this by an example. Here I am testing whether the MTU value 1480 is the proper value for my network. The ping command to perform this test is:

1. Here www.corenetworkz.com is the ping test destination
   
   
2. -l is used to switch the default ping packet size. You can learn more about it by following the link below.
   Use -l to Change Default Ping Packet Size
   
   
3. -f ensures the packets send are not fragmented.
   

See the test result below.

Microsoft Windows [Version 6.1.7600]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\sijugk>ping www.corenetworkz.com -f -l 1480

Pinging ghs.l.google.com [74.125.200.121] with 1480 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 74.125.200.121:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\Users\sijugk>

If the packet size you have entered is too high for your network, you will get the message Packet needs to be fragmented but DF set.

In order to solve the error, Packet needs to be fragmented but DF set, visit the link below and follow the steps mentioned on the tutorial.
Testing Proper MTU Size Of Your Network

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We all know the default size of ping ICMP echo request packet. That is 32 byte. Though the default ping packet size is 32 bytes, we can change it by a simple command while pinging a destination from your computer. By using -l option, you can decide the size of the echo request packets. The valid value of the size of ICMP packets is from 0 to 65500. So the size of the packet should fall in the range.

If you set any value, not in the range of 0-65500, you will get an error with warning bad value for -l, the value range is from 0 to 65500.

If you are facing issues with an MTU value of ICMP packets sent from your computer, changing their size may help you.

Let us check how to change the default packet size of ping echo request.

You need to use the command format provided below.

ping -l (value between 0 to 65500) [Destination Address]


Here let us check how to use this command to change the default size of ICMP echo request packets.

I am going to send packets to a destination address with manually configured value.

Here I use different values, and you can see the result on the screenshot provided below.

Example- ping -l 50 www.corenetworkz.com

Here I have changed the default value of 32 with 50.

Let us check how this command changes the size of echo packets send from your computer.


The above picture shows ping packets send with different sizes.

So I believe you have learned the procedure to assign any size for the ICMP request packets sent from your computer.

1. Destination Net Unreachable
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Likewise, if you want to change the default number of Echo requests send from your computer, you can do it by following the instructions provided in the link below.
How to Perform Continuous Ping Operation to a Destination Until Manually Stop

Not just the default size of the Ping requests but the TTL value too can be changed from your computer.

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If you want to change the default TTL value of the ICMP requests send from your computer like you have changed the default size, click on the link below.
How to Change Default TTL Value Set

1. PING: Transmit failed. General Failure
2. Ping Request Could not Find Host. Please Check the Name and Try again
3. Error 628: The Connection was Terminated by the Remote computer

Sometimes ping packets send from your computer with different packet sizes may bring you certain error messages instead of a proper reply from the destination.

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Most of the time it might be related to some other issues.

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The link provided below shows such an error.
Echo Reply Shows Hardware Error Instead Proper Reply

D-Link DSL-2520U ADSL2+ Ethernet/USB Combo Router is a high-speed ADSL Modem with routing feature. DSL-2520U can support up to 2 Mbps upstream and 24 Mbps downstream. You do not need extra Firewall protection If you are using DSL-2520U ADSL2+ Ethernet/USB Combo Router as ADSL modem. It is because DSL-2520U has its own built-in firewall. Another advantage of using D-Link DSL-2520U is the NAT firewall and SPI support for your network.

Since it is a modem cum router with DHCP feature, you do not need another device to share the Internet connection among multiple computers in your network. You can connect a computer to DSL-2520U ADSL2+ Ethernet/USB Combo Router with Ethernet cable(RJ 45), and USB cable.

Are You Facing Slower Internet With D-link?

Configure D-Link DSL-2520U ADSL2+ Ethernet/USB Combo Router

In order to configure D-Link DSL-2520U in your network, follow the instructions below.

1. Connect the Phone cable (DSL line) to the WAN/Internet port of D-Link DSL-2520U ADSL2+  as specified by the ISP
   
   
2. Connect D-Link DSL-2520U to Your Computer using an Ethernet cable
   
   
3. Power on the device and type 192.168.1.1 on the address bar of a browser and log in to the web-based setup page using following details.
   
   Username: admin
   Password: admin
   
   
4. Click on Quick Setup and check the box DSL Auto connect
   
   
   Click on Next
   
   
5. Enter the VCI/VPI values provided by your ISP to configure Internet connection on your DSL Modem
   
   
   Click Next
   
   
6. Select the connection type as specified by your ISP
   
   
7. Configure D-Link DSL-2520U ADSL2+ Modem in PPPoE
   
   If your ISP provides username and password to connect to the Internet, you should configure D-Link DSL-2520U ADSL2+ in PPPoE Mode. The advantage of configuring D-Link DSL-2520U ADSL2+ modem is that you can connect multiple computers to this device and share the Internet connection among them. Steps to setup D-Link DSL-2520U ADSL2+ modem in PPPoE mode is provided below.
   
   
   1. Select the connection type PPP over Ethernet(PPPoE)
      
      
      
      click on next.
      
      
   2. Enter the Username and password provided by your ISP
      
      
      
      click on next
      
      
   3. Check Enable NAT and Enable Firewall
       
      
      
      Click Next
      
      
   4. You must enable the DHCP server to connect multiple computers to this D-Link Modem. Modem IP address is 192.168.1.1 and Subnet Mask is 255.255.255.0
      
      
      
      Starting IP address is 192.168.1.2 and the end IP address is 192.168.1.254, Subnet Mask is 255.255.255.0. Now click next
      
      
   5. Now you are in the summary page.
      
      
      
      Click on save, and D-Link modem starts to reboot.
      
   
   
   
8. Configure D-Link DSL-2520U ADSL Modem in Bridge Mode
   
   If your ISP wants your modem configured in bridge mode, you can follow the steps mentioned here to setup D-Link DSL-2520U ADSL2+  in bridge mode. The issue with bridge mode is that you can connect only one computer to the D-Link DSL-2520U ADSL2+ modem and that computer will get the public IP address provided by ISP. Since there is no option to enable DHCP, you cannot use it as a router. You also need to use an external firewall if you are configuring D-Link DSL-2520U ADSL2+ modem in bridge mode because the built-in firewall is disabled in bridge mode.
   
   
   1. Select the connection type Bridging
      
      
      
      click Next
      
      
   2. Check Enable Bridge Service
      
      
      
      Click Next
      
      
   3. Enter the modem IP address as 192.168.1.1 and Subnet Mask 255.255.255.0
      
      
      
      Click Next
      
      
   4. Now we are at the final page of bridging D-Link ADSL Modem.
      
      
      
      Click Save. Now Dlink modem starts to reboot

Security Features of D-Link DSL-2520U

D-Link DSL-2520U ADSL2+ Ethernet/USB Combo Router is able to detect common attacks including DOS attack, port scan attack, etc.

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To learn more about DOS attack and how to prevent, click on the link below.
What is DoS Attack and How to Prevent it

If you have configured D-Link DSL-2520U ADSL2+ Ethernet/USB Combo Router in bridge mode, you cannot use the router feature of this device.

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You need an additional router to connect and share the Internet among multiple computers in the network. To configure a D-Link router, visit the link below.
How to Setup D-Link Wireless Router Manually