How to tell a nerd how to behave during the Corona cisis?!

The MIT CSAIL has tweeted a two-liner that resulted in quite a number of reactions - it is related to Covid-19 AND networking. Stay safe!

So this was tweeted by MIT CSAIL not long ago:

Stay at ??
Wear a ??????

Words of wisdom? Profanities? Gibberish?

If we have a look at the IANA address space registry, then 127/8 is reserved for loopback addresses. Also the IANA special-purpose address registry lists this address block. It was assigned in September 1981! Also RFC 1122 is referenced and it has the following to say about this prefix:

Internal host loopback address. Addresses of this form 
MUST NOT appear outside a host.

So addresses from this address block are to be used locally only. Packets going on the network must not use an address out of this prefix. But the Twitter post did not refer to the whole prefix but to alone. This is where RFC 5735 can help out with the following text: - This block is assigned for use as the Internet host
loopback address.  A datagram sent by a higher-level protocol to an
address anywhere within this block loops back inside the host.  This
is ordinarily implemented using only for loopback.  As
described in [RFC1122], Section, addresses within the entire block do not legitimately appear on any network anywhere.

So typically, out of the whole /8, with its over 16 million addresses, only is typically used… That seems wasteful. But this particular address has also a particular name. Let’s try what dig has to say about this. Let’s do a reverse DNS lookup of that address:

dig -x

; <<>> DiG 9.10.6 <<>> -x
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 9500
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

; EDNS: version: 0, flags:; udp: 1444

;; ANSWER SECTION:	10800	IN	PTR	localhost.

;; Query time: 5 msec
;; WHEN: Thu Dec 10 18:57:30 CET 2020
;; MSG SIZE  rcvd: 74

So this IP address is referred to localhost! And indeed it is used in various nerd merchandise. You can e.g. buy doormats that read:

There is no place like

There is no place like home. So the first line of the Tweet reads: Stay at home. That’s good solid Covid-19 advice! But what about the second line here: Wear a

The IANA address space registry has 255/8 registered for future use. You cannot really wear the future. But when you look at the special address and when you realize that this special address comes out of a special /8, then you might remember a different way to write down how many bits belong to the network prefix. That’s right, the subnet mask, a bitmask that, if you logically AND it with an address gives you the network prefix! So the subnet mask corresponding to a /8 would be

So please wear a mask! And if you follow both pieces of advice, you’ll hopefully stay healthy during the Covid-19 pandemic.

Of course the target audience did not leave it at that. E.g. it was noted that really IPv6 should have been used as an example. After all this is 2020. The loopback address in IPv6 is really just an address. There is no prefix for local use. The address is ::1/128. This wouldn’t really work out though, as the subnet mask notation is not used in IPv6 (thankfully). Others have noted that with that subnet mask, there would really be too many people allowed to share the same home. Suggestions to use a prefix that allows less hosts were therefore made. And it went on and on and on… Well, what did the folks at MIT expect would happen.

Stay healthy everyone!

Rolf Winter
Rolf Winter
Professor of Computer Networks

My research interests include everything about networks starting at L2.