# Important Diagrams - Computer Networks ## OSI Model Data Flow Diagram Description The OSI model shows data flowing down through seven layers on the sender side and up through seven layers on the receiver side. At each layer on the sender a header is added to the data. At each layer on the receiver the corresponding header is removed. The physical layer at the bottom connects both sides through the transmission medium. The layers from top to bottom on the sender side are Application, Presentation, Session, Transport, Network, Data Link, Physical. The layers from bottom to top on the receiver side are Physical, Data Link, Network, Transport, Session, Presentation, Application. ## TCP Three Way Handshake Diagram Description The diagram shows two columns, one for Client and one for Server, with a timeline going downward. The Client sends SYN with sequence number x to the Server. The Server responds with SYN ACK, acknowledging x plus 1 and sending its own sequence number y. The Client responds with ACK acknowledging y plus 1. After this exchange the connection is shown as ESTABLISHED on both sides. ## Ethernet Frame Structure Description An Ethernet frame from left to right contains the Preamble which is 7 bytes, the Start Frame Delimiter which is 1 byte, the Destination MAC Address which is 6 bytes, the Source MAC Address which is 6 bytes, the Type or Length field which is 2 bytes, the Data payload which is 46 to 1500 bytes, and the Frame Check Sequence CRC which is 4 bytes. ## IPv4 Header Structure Description The IPv4 header from left to right and top to bottom contains Version (4 bits), Header Length (4 bits), Type of Service (8 bits), Total Length (16 bits), Identification (16 bits), Flags (3 bits), Fragment Offset (13 bits), Time to Live (8 bits), Protocol (8 bits), Header Checksum (16 bits), Source IP Address (32 bits), Destination IP Address (32 bits), and optional Options field. ## TCP Header Structure Description The TCP header contains Source Port (16 bits), Destination Port (16 bits), Sequence Number (32 bits), Acknowledgment Number (32 bits), Data Offset (4 bits), Reserved (3 bits), Flags (9 bits) including URG ACK PSH RST SYN FIN, Window Size (16 bits), Checksum (16 bits), Urgent Pointer (16 bits), and optional Options. ## Subnetting Diagram Description A subnetting diagram shows a large network block such as 192.168.1.0/24 at the top. Below it arrows point to four smaller subnet blocks such as 192.168.1.0/26, 192.168.1.64/26, 192.168.1.128/26, and 192.168.1.192/26. Each subnet block shows its network address, usable host range, and broadcast address. ## Network Topology Diagrams Description Bus topology shows all devices connected to a single horizontal line with terminators at each end. Star topology shows a central switch with lines radiating outward to devices in all directions. Ring topology shows devices arranged in a circle with each device connected to its two neighbors. Mesh topology shows every device connected to every other device with multiple crossing lines. Tree topology shows a root node at top, branch nodes in middle, and leaf nodes at bottom. ## OSI vs TCP/IP Model Comparison Description Two parallel columns show the models side by side. The OSI column shows seven layers from top to bottom: Application, Presentation, Session, Transport, Network, Data Link, Physical. The TCP/IP column shows four layers: Application, Transport, Internet, Network Access. Brackets show that the TCP/IP Application layer maps to OSI layers 7, 6, and 5. TCP/IP Transport maps to OSI layer 4. TCP/IP Internet maps to OSI layer 3. TCP/IP Network Access maps to OSI layers 2 and 1. ## DHCP Process Diagram Description The diagram shows a Client on the left and a DHCP Server on the right with arrows between them. Arrow 1 goes from Client to Server labeled DHCP Discover with broadcast address 255.255.255.255. Arrow 2 goes from Server to Client labeled DHCP Offer with proposed IP address. Arrow 3 goes from Client to Server labeled DHCP Request accepting the offer. Arrow 4 goes from Server to Client labeled DHCP Acknowledge confirming the assignment.