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wtupc96 · Apr 16, 2017 · 98e6f16 · 98e6f16
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diff --git a/2/2Computer_network.tex b/2/2Computer_network.tex
@@ -17,6 +17,7 @@ \section{Additional Problems}
 
 	\item Specify following system Organization:
 	\subitem Email system, DNS system
+	\subfile{2ans.tex}
 \end{enumerate}
 
 \section{Problems on textbooks}

diff --git a/2/2ans.tex b/2/2ans.tex
@@ -1 +1,6 @@
 \paragraph{A.}
+\begin{enumerate}
+	\item Email system has two basic components: User-agents and Email server. The user-agents can send messages to the Email server with any protocols, then the messages will be sent to the destination Email server with SMTP, finally the receiver gets your message from the destination server using protocol like POP3 and IMAP.
+
+	\item DNS system is a distributed database implemented in a hierarchy if DNS servers and an application-layer protocol that allows hosts to query the distributed database. At the top of the hierarchy is Root DNS servers and below it there are com DNS servers and org DNS servers and edu DNS servers and so on. All in all, there are three classes of DNS servers: Root DNS servers, Top-level domain servers and Authoritative DNS servers.
+\end{enumerate}