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+---
+title: "11-DNS"
+tags: 
+- lecture
+- cosc301
+---
+
+> [!INFO] 
+> hosts are named by ip addresses: e.g., 192.168.1.2
+> we want to map an ip address to a readable name so it is memorable
+> in the past it was easy to store all the name of possible address in a folder `/etc/hosts`. 
+> it uses a NIC networking inforamtion system. 
+> but this is only for LAN
+> impossible to store all IP addresses in the internet this way
+
+
+> [!INFO] 
+> DNS resolves a name to an IP address. can also do reverse mapping (this is useful for detecting spam emails).
+
+> [!INFO] distributed dns
+> now dns is distributed. it is a BIND (berkely internet name domain) service. 
+> to do something distributedly. each LAN has their own DNS
+
+Problem  
+- How to get the IP address with an IP name?  
+	- Mapping between IP addresses and IP names  
+- Simple solution  
+	- Central database, like /etc/hosts or Sun  
+Microsystems’ NIS (Network Information  
+Service) or Windows’ WINS for LAN.  
+	- E.g. tkm.govt.nz 13.77.63.24  
+- However, keeping billions of such records  
+in a central database used by billions of  
+users is almost impossible.
+
+Domain Name Service (DNS)  
+- A distributed solution  
+- Each organisation, called domain, maintains  
+its own database and answers queries about  
+its domain.  
+- E.g.  
+	- oucs1120.otago.ac.nz 139.80.22.10  
+	- oucs1234.otago.ac.nz 139.80.34.67  
+	- chasm.otago.ac.nz 139.80.45.90  
+	- hextreme.otago.ac.nz 139.80.63.88
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