Add NAT links

Author: theEmbeddedGeorge

Network/icmp.md

@@ -21,6 +21,7 @@ first place (so that the sender can determine the datagram that caused the error
 
 ## Application
 
+
 - **Ping**: The well-known ping program sends an ICMP type 8 code 0 message to the
 specified host. The destination host, seeing the echo request, sends back a type 0
 code 0 ICMP echo reply
@@ -38,6 +39,20 @@ Because this datagram contains a UDP segment with an unlikely port number, the
 destination host sends a port unreachable ICMP message (type 3 code 3) back to the
 source.
 
+### How Does Ping Work?
+
+1. As the ping program initializes, it opens a raw ICMP socket so that it can employ IP directly, circumventing TCP and UDP.
+2. Ping formats an ICMP type 8 message, an Echo Request, and sends it (using the “sendto” function) to
+the designated target address. The system provides the IP header and the data link layer envelope.
+3. As ICMP messages are received, ping has the opportunity to examine each packet to pick out those
+items that are of interest.
+4. The usual behavior is to siphon off ICMP type 0 messages, Echo Replies, which have an identification
+field value that matches the program PID.
+
+[How Does Ping Really Work](http://www.galaxyvisions.com/pdf/white-papers/How_does_Ping_Work_Style_1_GV.pdf)
+
+This inspirational document detailed about how the well-know ping application works. 
+
 ## Reference
 
 https://www.geeksforgeeks.org/internet-control-message-protocol-icmp/

Network/nat.md

@@ -1,6 +1,6 @@
 ## NAT
 
-**Network address translation (NAT)** is a method of remapping an IP address space into another by modifying network address information in the IP header of packets while they are in transit across a traffic routing device.[1] The technique was originally used to avoid the need to assign a new address to every host when a network was moved, or when the upstream Internet service provider was replaced, but could not route the networks address space. It has become a popular and essential tool in conserving global address space in the face of IPv4 address exhaustion. One Internet-routable IP address of a NAT gateway can be used for an entire private network.
+**Network address translation (NAT)** is a method of remapping an IP address space into another by modifying network address information in the IP header of packets while they are in transit across a traffic routing device. The technique was originally used to avoid the need to assign a new address to every host when a network was moved, or when the upstream Internet service provider was replaced, but could not route the networks address space. It has become a popular and essential tool in conserving global address space in the face of IPv4 address exhaustion. One Internet-routable IP address of a NAT gateway can be used for an entire private network.
 
 ![NAT translation](images/nat_translation.png)
 
@@ -27,6 +27,23 @@ T- ranslation results in switching path delays.
 
 ## Reference
 
+[Introduction to Network Address Translation](https://nsrc.org/workshops/2018/btnog-wireless/presentations/00_NAT_Introduction.pdf)
+
+This slide provide a comprehensive overview of NAT and illustrate its typical use case with Campus Network Design & Operations Workshop.
+
+[Cisco − How NAT Works](http://academy.delmar.edu/Courses/download/CiscoIOS/NAT_HowItWorks.pdf)
+
+A document published by Cisco that gives a insighful angle of how NAT works.
+
+[A Retrospective View of Network Address Translation](http://web.cs.ucla.edu/~lixia/papers/08IEEE-NAT-Retrospect.pdf)
+
+Today, network address translators, or NATs, are everywhere. Their ubiquitous
+adoption was not promoted by design or planning but by the continued growth of
+the Internet, which places an ever-increasing demand not only on IP address space
+but also on other functional requirements that network address translation is perceived to facilitate. This article presents a personal perspective on the history of
+NATs, their pros and cons in a retrospective light, and the lessons we can learn
+from the NAT experience.
+
 [NAT WIKIpedia](https://en.wikipedia.org/wiki/Network_address_translation)
 
 [NAT Geek for geek](https://www.geeksforgeeks.org/network-address-translation-nat/)

README.md

@@ -382,6 +382,7 @@
 ### E. [Brain Teaser](https://www.geeksforgeeks.org/puzzles/)
 
 ### F. System Design
+1. [System Design Primer](https://github.com/donnemartin/system-design-primer)
 
 ### G. By Company
 1.  [Tesla](Interview/Company/tesla.md)