networking: Turn homework on NAT into a subsection.

diff --git a/Networking.m4 b/Networking.m4
index f64b341100251d76559d04357ba9d79b046ca12e..c0bbd2b8fe3256b9e882249d77dac886f7b409a7 100644 (file)
--- a/Networking.m4
+++ b/Networking.m4
@@ -256,6 +256,44 @@ of the packet. </p>
 facility, which means that packets may be lost, arrive multiple times,
 or out of order. Moreover, packets can be fragmented or defragmented. </p>
 
+SUBSECTION(«Network Address Translation (NAT)»)
+
+<p> IP addresses are a global resource, so each address must only be
+assigned to at most one device. The <em>Internet Assigned Numbers
+Authority</em> (IANA) coordinates who is entitled to use any given
+range of IPv4 addresses. However, a number IP ranges, for example the
+ranges 10.0.0.0-10.255.255.255 and 192.168.0.0-192.168.255.255, are
+reserved for local use only so that these addresses can be assigned
+in multiple local networks without interfering with each other. </p>
+
+<p> In a network which contains only local addresses, the devices
+can communicate with each other, but they will not be able to access
+public world-wide services. To illustrate how NAT works around this
+restriction, consider a desktop computer in a local network which
+intents to perform a web search by contacting a public web server
+which is <em>not</em> a member of the local network. </p>
+
+<p> The desktop is configured to route packets which are not
+destined for the local network through a dedicated machine, called
+the <em>router</em>. The router has two IP addresses: one address
+in the local network and a public NAT address. As traffic passes
+from the desktop through the router to the web server, the router
+alters the source address of each IP packet (the local address of the
+desktop) to the public NAT address of the router, and keeps track of
+all thusly forwarded connections. When a reply arrives at the router,
+it uses the connection tracking data stored during the outbound phase
+to determine the address in the local network to which to forward the
+reply. This time the router alters the <em>destination</em> address
+of the packet to the local address of the desktop and forwards the
+packet to the desktop via the local network. </p>
+
+<p> NAT can be seen as providing a kind of privacy mechanism because
+machines on the internet cannot monitor which hosts are sending and
+receiving traffic. They only see the NAT address.    NAT has also
+downsides though: Pinpointing the source of a problem becomes harder,
+and encryption becomes more difficult. For example you can not encrypt
+the IP address because the router must be able to change it. </p>
+
 EXERCISES()
 
 <ul>
@@ -295,37 +333,6 @@ to which potential attackers have physical access, i.e., untrusted
 devices can be connected.
 »)
 
-HOMEWORK(«
-
-Illustrate how <em> network address translation </em> (NAT) works
-on the basis of a web search initiated from a desktop computer in a
-local network and discuss the implications that NAT has on privacy.
-
-», «
-
-<p> The desktop is configured to route packets which are not destined
-for the local network through a dedicated machine, called the <em>
-router</em>. In particular, all internet traffic is sent to the router.
-The router has two IP addresses: one address in the local network
-and a public NAT address. As traffic passes from the desktop through
-the router to the web server in the internet, the source address of
-each IP packet (the local address of the desktop) is changed on the
-fly to the public NAT address of the router. The router tracks each
-active connection. When a reply arrives at the router, it uses the
-connection tracking data stored during the outbound phase to determine
-the address in the local network to which to forward the reply. This
-time it overwrites the destination address of the IP packet with the
-local address of the desktop. </p>
-
-<p> NAT can be seen as providing a kind of privacy mechanism because
-machines on the internet cannot monitor which hosts are sending and
-receiving traffic. They only see the NAT address.    NAT has also
-downsides though: Pinpointing the source of a problem becomes harder,
-and encryption becomes more difficult. For example you can not encrypt
-the IP address because the router must be able to change it. </p>
-
-»)
-
 HOMEWORK(«
 Run <code> tracepath wikipedia.org</code>. Explain how this command
 works and how it can be used to identify networking problems.
@@ -433,11 +440,10 @@ HOMEWORK(«
 
 SECTION(«Application Layer»)
 
-<p> Application layer protocols define how the server side of a network
-service communicates with clients that connect to the server by
-connecting a specific TCP or UDP port. Services are often associcated
-with port numbers which can be registred at the <em> Internet Assigned
-Numbers Authority </em> (IANA). </p>
+<p> Application layer protocols define how the server side of
+a network service communicates with clients that connect to the
+server by connecting a specific TCP or UDP port. Services are often
+associcated with port numbers which can be registred at the IANA. </p>
 
 <p> Examples for application layer protocols which are employed on top of
 TCP are the <em> Hypertext Transfer Protocol </em> (HTTP, port 80)