DNS: how does the world find my nameservers?

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http://serverfault.com – I have a domain, mydomain.com.au. I make two nameservers for this domain, ns1.mydomain.com.au and ns2.mydomain.com.au, and load them full of NS records, A records, MX records and anything else I feel like hosting. Now, how do I get these to be used? If I go to my registrar and say the nameservers for mydomain.com.au are ns1.mydomain.com.au and ns2.mydomain.com.au how is anyone going to find the IPs to connect to lookup addresses, since the only systems in the world that know the public IPs for my nameservers are... my nameservers. I'm missing something obvious here, please let me know what (HowTos)