Arp-a results on Terminal

224.0.0.251 is the mDNS multicast address (Domain Name Server, i.e. translate IP address to named address).

https://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml#multicast-addresses-1

I don't know what 239.255.255.250 is except as noted here: https://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml#multicast-addresses-12

And, here is the RFC for Administratively-scoped IP Multicast. https://www.rfc-editor.org/rfc/rfc2365, but I can't make anything of it.

Yes, mDNS is certainly normal. I don't have any administratively scoped multicast addresses, though. What prompted you to enter the arp command?

joanna0397 wrote:

Hello --

Could someone explain what the following addresses are in my network that I found on Terminal via the arp -a command:

224.0.0.251 at 1:0:5e:0:0:fb on en0 ifscope permanent [ethernet]

239.255.255.250 at 1:0:5e:7f:ff:fa on en0 ifscope permanent [ethernet]

Through a Google search I was able to see that they are multicast addresses, but still don't know what they are or how they got onto my network? Is this normal to have? Did it happen automatically? Does it pose an issue?

Seems normal— is there some associated issue? Why query arp...(?)

DESCRIPTION

The arp utility displays and modifies the Internet-to-Ethernet address translation tables used by the

address resolution protocol

I have never used an Ethernet connection on this MacBook Pro

for example this is what I see—

MacBook-Pro ~ % arp -a

? (172.20.10.1) at b6:56:e3:3b:dc:64 on en0 ifscope [ethernet]

? (172.20.10.2) at a4:83:e7:cb:72:9c on en0 ifscope permanent [ethernet]

? (172.20.10.15) at ff:ff:ff:ff:ff:ff on en0 ifscope [ethernet]

? (224.0.0.251) at 1:0:5e:0:0:fb on en0 ifscope permanent [ethernet]

broadcasthost (255.255.255.255) at ff:ff:ff:ff:ff:ff on en0 ifscope [ethernet]

MacBook-Pro ~ %

I'm pretty sure that en0 is the device name for your network interface. If you really want to see what your computer is connecting to issue this command in terminal:

Look mostly at Local Address and Foreign Address

netstat -an | more

on terminal