Problem/bug with NS records

[Diaz Rodriguez, Eduardo]

ok, i have a one domain in Internet and this is my file:

=================domain.com========================
$TTL 172800
@                                IN SOA 62.37.3.56. postmaster.domain.com. (
                                  2000102304          ; Serial para
actualizaciones
                                  86400      ; Refresco despues de 1 dia
                                  7200       ; Reintento despues de 2 horas
                                  172800    ; Expira despues de 2 dias
                                  172800 )   ; TTL minimo de 2 dias
;
; Insercion por DNS automatica
;
@        IN      NS      62.37.3.56.
;Direcciones de hosts
@       172800  IN MX   10 62.37.3.39.
@       172800  IN A     62.37.3.13
www     172800  IN A     62.37.3.13
back    172800  IN A     62.37.3.66
@       172800  IN MX   20 62.37.3.66.
@       172800  IN NS    62.37.3.57.
=================domain.com========================

i can make nslookup for all machines in my domain in my name server.
but i make nslookup with server of yahoo or another name server "for
example" back.domain.com, the server can't find, but the server can find
www.domain.com.

ok, the next is, if i change NS resource records for names " not for ip's" ,
and chage the SOA ip for names, the system run ok.

where is the problem?, is it a bug?, there isn't one IP as same as name?.

thank for all

Eduardo Díaz 
Técnico Sistemas de información de Red 
E-Mail: ediaz at uni2.es