ajout 10 IPv4 Header

00. Jeremy's IT LAB CCNA course.md

@@ -18,3 +18,4 @@ tags:
 [[Ethernet LAN Switching (part 2)]]
 [[IPv4 Addressing (Part 1)]]
 [[IPv4 Addressing (Part 2)]]
+[[IPv4 Header]]

09. Switch Interfaces.md

@@ -13,8 +13,7 @@ To see all the [[Switch]] interfaces in the CLI use
 
 ```cisco
 SW1>en
-SW1#show ip interface bief
+SW1#show ip interface bief ```
-```
 
 [[Router]] interfaces have the shutdown command applied by default
   - will be in the administratively down/down state by default

10. IPv4 Header.md

@@ -0,0 +1,151 @@
+---
+id: 1777370971-DYNV
+aliases:
+  - IPv4 Header
+tags: []
+---
+
+# IPv4 Header
+
+
+## OSI Model - [[PDU]]s
+
+Application Data
+↓
+Data + L4 Header = Segment
+↓
+Segment + L3 Header = Packet
+↓
+Packet + L2 Header + L2 Trailer = Frame
+
+We will focus on the Packet for this lesson
+
+## IPV4 header
+
+### Version Field
+
+length: 4 bits (half of on octet)
+- identifies the version of IP used.
+- IPv4 = 4 (0b0100)
+- IPv4 = 6 (0b0110)
+
+### Internet Header Length (IHL)
+
+length: 4 bits (half of on octet)
+
+- The final field of the IPv4 header (Options) is variable in length, so this field
+is necessary to indicate the total length of the header
+- identifies the lenght of the header in **4 byte increments**
+- Value of 5 = 5*4 bytes = 20 bytes
+- minimum value is 5 (= 20 bytes)
+  - **Minimum IPv4 Header Length = 20 bytes**
+- Maximum value is 15 (15*4 bytes = 60 bytes)
+  - **Maximum IPv4 Header Length = 60 bytes**
+
+### DSCP field
+
+- Length: 6 bits
+- Differentiated Services Code Point
+- Used for Qos (Quality of Service)
+- Used to prioritize delay-sensitive dta (streaming voice, video, etc..)
+
+### ECN field
+
+Explicit Congestion Notification
+- Length: 2 bits
+- Provides end-to-end (between two endpoints) notification of network congestion 
+**without dropping packets**
+- Optional feature that requires both endpoints, as well as the underlying network 
+infrastructure, to support it.
+
+### Total Length field
+
+Length: 16 bits
+- Indicates the total lenght of the packet (L3 header+ L4 segment)
+- Measured in bytes (**not 4-byte increments like IHL**)
+- Minimum value of 20 (=IPv4 header with no encapsulated data)
+- Maximum value of 65,535 (maximum 16-bit value)
+
+### Identification field
+
+Length: 16 bits
+- if a packet is fragmented due to being too large, this field is used to identify
+which packet the fragment belongs to
+- All fragments of the same packet will have their own [[IPv4]] 
+header with the same value in this field
+- Packets are fragmented if larger than the MTU (Maximum Transmission Unit)
+- The MTU is usually 1500 bytes
+- Fragments are reassembled by the receiving host
+
+### Flags field
+
+Length: 3bits
+
+- Used to control/identify fragments.
+- Bit 0: Reserved, always set to 0
+- Bit 1: Don't Fragment (DF bit), used to indicate a packet that should not be fragmented
+- Bit 2: More Fragments (MF bit), set to 1 if there are more fragments in the packet,
+set to 0 for the last fragment
+
+*Unfragemented packets will always have their MF bit set to 0*
+
+### Fragment Offset field
+
+Length: 13 bits
+
+- Used to indicate the position of the fragment within the original, unfragmented IP packet
+- Allows fragmented packets to be reassembled even if the fragments arrive out of order
+
+### Time to live field
+
+Length: 8 bits
+- A router will drop a packet with a TTL of -
+- Used to prevent infinite loops
+- Originlly designed to indicte the packet's maximum lifetime in second
+- In practice, indicates a 'Hop count': each time the packet arrives at a router, 
+the router decreases the TTL by 1
+*Recommended default TTL is 64*
+
+### Protocol field
+
+Length: 8 bits
+
+- Indicates the protocol of the encapsulated L4PDU
+- Value of 6: [[TCP]]
+- Value of 17: [[UDP]]
+- Value of 1: [[ICMP]]
+- Value of 89 : [[OSPF]] dynamic routing protocol
+
+### Header Checksum field
+
+Lenght 16bits
+
+- A calculaed checksum used to check for errors in the IPv4 header.
+- When a router receives a packet, it calculates the checksum of the header 
+and compares it to the one in this field of the header
+- if they do not match the router drops the packet
+- Used to check for errors only in the IPV4 header.
+- IP relies on the encapsulated protocol to detect errors in the encapsulated data
+- Both TCP and UDP have their own checksum fields to detect errors in the encapsulated data
+
+### Source/Destination IP address fields 
+
+Length 32 bits(each)
+
+- Source IP Address = IPv4 address of the sender of the packet
+- Destination IP Address = IPv4 address of the intended receiver of the packet
+
+### Options fields
+
+Length: 0 - 320 bits
+
+- Rarely used.
+- If the IHL field is greater than 5, it means that Options are present
+
+
+
+
+## Review
+
+- IPv4 Packet structure
+- Fields of the IPv4 header