newbie question

[Masaki Nishikawa]

I'm setting up a zone file for IP-reverse lookup.

A db file looks like this.
(The actual IP address and the domain name are masked by "xxx").

> $ORIGIN xx.xxx.in-addr.arpa.
> xxx             IN      SOA     ns-j01.xxx.net. root.xxx.net. (
>                 1 10800 3600 604800 86400 )
>                 IN      NS      ns-j01.xxx.net.
>                 IN      NS      ns-j02.xxx.net.

But when I start named daemon, I get the following message on all the db
files I created.

> Jun 14 16:29:43 ns-j01.xxx.net named[17896]: Zone "xxx.xx.xxx.in-addr.arpa" (file
> db.xxx.xx.xxx): No default TTL set using SOA minimum instead

What does it mean by "No default TTL set"????
In the above db file, the TTL is 86400, right???

Thanks,

Masaki