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Brief OSI Model
- OSI Layer Control information and Functions:
|Data||7. Application||* Supports Application and End-User processes.
* Provides Application services for file transfers, e-mail, and other network software services.
|HTTP, FTP, SMTP, SSH, TELNET|
|6. Presentation||* Provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa.
* Works to transform data into the form that application layer can accept.
* This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems.
|HTML, CSS, GIF|
|5. Session||* Establishes, manages and terminates connections between applications.
* Sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end.
* It deals with session and connection coordination.
|RPC, PAP, SSL, SQL, NetBIOS,|
|Segments||4. Transport||* Provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control.
* It ensures complete data transfer.
|TCP, UDP, SCTP|
|Packet/Datagram||3. Network||* Provides routing technologies and handles logical addressing.
* Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.
|IPv4, IPv6, IPsec, AppleTalk, ICMP|
|Bit/Frame||2. Data link||* Handles Physical Addressing.
* Data packets are encoded and decoded into bits.
* It is divided into two sub layers: Media Access Control (MAC) layer and Logical Link Control (LLC) layer.
* The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it.
* The LLC layer controls frame synchronization, flow control and error checking.
|PPP, IEEE 802.2, L2TP, MAC|
|Bit||1. Physical||* Conveys the bit stream - electrical impulse, light or radio signal — through the network at the electrical and mechanical level.
* Provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects.
|DSL, USB, ISDN, RS-232|
- A network that is formed from the combination of two or more networks (or subnets) with a common Classless Inter-Domain Routing (CIDR) prefix.
- It must not contain other prefixes of networks that do not lie in the same routing path.
- Also called Supernetting, Prefix Aggregation, Route Aggregation, or Route Summarization.
- Steps for calculating a Supernet
192.168.98.0 192.168.99.0 192.168.100.0 192.168.101.0 192.168.102.0 192.168.105.0
- Addresses are converted to binary format:
|Address||First Octet||Second Octet||Third Octet||Fourth Octet|
- Bits at which the common pattern of digits ends are located.
- The number of common bits is counted.
- The summary route is found by setting the remaining bits to zero,
- It is followed by a slash and then the number of common bits.
|First Octet||Second Octet||Third Octet||Fourth Octet||Address||Netmask|
- The summarized route is 192.168.96.0/20. The subnet mask is 255.255.240.0.
- This summarized route also contains networks that were not in the summarized group:
192.168.96.0 192.168.97.0 192.168.103.0 192.168.104.0 192.168.106.0 192.168.107.0 192.168.108.0 192.168.109.0 192.168.110.0 192.168.111.0
- It must be assured that the missing network prefixes do not exist outside of this route.
- An ISP is assigned a block of IP addresses by a regional Internet registry (RIR) of 220.127.116.11 to 18.104.22.168.
- The ISP might then assign subnetworks to each of their downstream clients, e.g:
Customer A will have the range 22.214.171.124 to 126.96.36.199 Customer B would receive the range 188.8.131.52 to 184.108.40.206 Customer C would receive the range 220.127.116.11 to 18.104.22.168, and so on.
- Instead of an entry for each of the subnets 172.1.1.x and 172.1.2.x, etc., the ISP could aggregate the entire 172.1.x.x address range and advertise the network 22.214.171.124/16 on the Internet community, which would reduce the number of entries in the global routing table.
- OSPF is a layer 4 protocol
Encapsulated in an IP Packet(Protocol no 89) Uses Acknowledgement
- RIP is a layer 7 protocol(uses UDP port 512)
- BGP is a layer 7 protocol (uses Port no 179)
- A PC can have only 1 Default Gateway configured. Other Interfaces will generally not have any default gateway.
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