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Network+_OSI.md

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+# __Network +__ 
+
+## __OSI Model__
+## __Layer 1__ Bites 
+  * Physical Layer 
+    * It is the connents like Cat 6 RJ45 and fiber.
+    * Its the on and off on the cables.
+---
+
+## __Layer 2__ Frames
+* ### Data Link Layer.
+    * Packagers data int frames and transmits those frames on the network.
+    * Layer 2 devices view nteworks __logically__.
+* ### MAC Address 
+    * Media Access Control
+    * Physical addressing system of a device which operates on the logial topology.
+    * Uses a 48-bit address assigned to a network interface card. 
+    * __EX:__ `D2:51:F1:3A:34:65` 
+    * `D2:51:F1` is the vedor code 
+* ### Logical Link Control
+    * Provides connection services and allows acknowledgemnt of receipt of message.
+    * Is the basic form of flow control.
+    * Provides basic error control functios.
+    * Uses a check sum. 
+* ### How communication synchronized.
+    * Isochronous
+        * Netwok devices use a common reference a common refernce clock source and create time slots for transmission.
+    * Synchronous 
+        * Network devices agree on clocking method to indicate beginning and end of frames and can use control characters. 
+    * Asynchonous
+        * Network devices reference thier own internal clocks and use start and stop bits 
+* ### What Devices are layer 2
+    * network cards
+    * Briges
+    * Switch
+    * MAC Address
+---
+## __Layer 3__ Packets
+
+* ### Network Layer
+    * Forwards traffic (routing) with logical address. 
+    * __IP address__ IPv4 IPv6
+    * Logical addressing 
+        * Ip Addressing  IPv4 and IPv6
+    * Switching (Routing)
+    * Route discovery and selction
+    * Connection Services 
+    * Banwidth usage
+    * Multiplexing
+* ### How should data be __forward__ or __routed__ 
+    * Packet Switching (Most common)
+        * Data is divided into packets and then forwarded to is IP address.
+    * Circuit Switching
+        * Dedicated communication link is establed between two devices.
+        * It uses one path when the connect establed and the whole time the connect is there. 
+    * Message Switching
+        * Data is divided into messages which may be stored and then forwarded. 
+* ### Route Discovery and Selection
+    * Manually configured as a static route or dynamically through.
+* ### Connection Services
+    * Augment Layer 2 connection services to improve reliability
+    * Flow Control 
+    * Packet reordering
+* ### Internet Control Message Protocol
+    * Sends error messages and operational information to an IP destination.
+        * `Ping`
+        * Trace Route `tracert`
+* ### Devices 
+    * __Routers__ 
+    * Multilayer switches
+* ### Protocol 
+    * __IPv4__
+    * __IPv6__ 
+    * ICMP
+
+---
+
+## __Layer 4__ Segments
+* ### Transport Layer
+    * TCP Transmission Control Protocol
+        * Connection-oriented protocol that is a reliable way to transport segments across the network 
+        * Will ask for an acknowledgement  
+        * ![TCP](./pics/tcp.png) 
+        * This is a Three-way handshake.
+    
+
+    * UDP User Datagram Protocol
+        * Connectionless protocol that is an unreliable way to transport segments across the network 
+
+    * Windowing
+    * Buffering