Networking - Well i never knew that........

I am starting this thread for people to post interesting facts about general networking functions that when you found out you sat there and thought "Oh Yeah" or "Well I Never" or something along them lines.

I thought it may be worth sharing this between everyone so they can also experience the same

PLEASE DONT CLOG THIS THREAD UP WITH RESPONSES TO POST, ONLY POST YOUR "Well I Never Knew That" EXPERIENCES

So, i will start it off then.

As i am preparing for my CCNP - Switching exam, i read something that i always wondered about but never actually realised the why. I will share this moment with everyone.

This is to do with ARP;

Have you ever done a ping to a host and the first request came back "Request Timed Out" and then the others came back ok. From then on it works ok ? Have you ever wondered why ?

Well, here ya go. Apparently its to do with the ARP RFC. If the gateway of the Remote Host (i.e. the router that is connected to the remote network) doesn't have the ARP entry in its table/cache. The router will drop all traffic to stop it queuing, until its been through the ARP process to get the layer 2 MAC address.

For this reason, the first request will time out and then after that time, the router knows the MAC address and will therefore then start to send the packets to the remote client (or its local client).

Hope others have something to share

Good thread so I will give it a little /bump.

One thing that surprised me recently was when I was getting into multicasts. Depending on the actual milticast IP used you can find out all sorts of fun things.

>Ping 224.0.0.1 will get every device on that subnet to respond to the ping request. You won't see their individual replys in the dos window but if you are packet tracing you will see all devices reply. (as long as they don't have ICMP disabled that is)

There are loads of others for routers to respond, routing protocols to check in with each other, etc.