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Good day, I connected two computers together with an Ethernet straight-through cable. I did not configure any ip address, and after a while they both got APIPA addresses. Now they can ping each other and share files. I am a bit confused because firstly, it's a straight-through cable, so there should be no communication and secondly, when I type in "ipconfig /all" in cmd on both computers, there is no subnet mask. How is communication even possible? I am so confused.

  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 12 '17 at 18:06
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Nowodays, many ethernet interfaces are 'auto MDI/MDIX'.

They detect the type of device connected and if needed reverse internally Transmit and Receive.

This allow connection of 2 devices of the same layer with straight-through cable.

APIPA is specifically made to provide IP addresses and allow IP communication in a network where there's no DHCP server.

There's a subnet mask associated with APIPA-assigned IP addresses, it is 255.255.0.0 (I.E. /16). On a windows 7 machine this subnet mask is shown by "ipconfig /all".

Maybe windows 8 or 10 hide it because it's implicit with APIPA, but I cannot check it right now.

  • The 169.254.0.0/16 network is not allowed to be subnetted. The mask will always be /16 (255.255.0.0). – Ron Maupin Sep 16 '15 at 14:19

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