OSI Model
Brief OSI Model
- OSI Layer Control information and Functions:
OSI Model | ||||
---|---|---|---|---|
PDU | Layer | Function | Examples | |
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 |
SuperNetting
- 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 |
---|---|---|---|---|
192.168.98.0 | 11000000 | 10101000 | 01100010 | 00000000 |
192.168.99.0 | 11000000 | 10101000 | 01100011 | 00000000 |
192.168.100.0 | 11000000 | 10101000 | 01100100 | 00000000 |
192.168.101.0 | 11000000 | 10101000 | 01100101 | 00000000 |
192.168.102.0 | 11000000 | 10101000 | 01100110 | 00000000 |
192.168.105.0 | 11000000 | 10101000 | 01101001 | 00000000 |
- 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 |
---|---|---|---|---|---|
11000000 | 10101000 | 01100000 | 00000000 | 192.168.96.0 | /20 |
- 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.
- Example
- An ISP is assigned a block of IP addresses by a regional Internet registry (RIR) of 172.1.0.0 to 172.1.255.255.
- The ISP might then assign subnetworks to each of their downstream clients, e.g:
Customer A will have the range 172.1.1.0 to 172.1.1.255 Customer B would receive the range 172.1.2.0 to 172.1.2.255 Customer C would receive the range 172.1.3.0 to 172.1.3.255, 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 172.1.0.0/16 on the Internet community, which would reduce the number of entries in the global routing table.
Misc
- 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.
- References
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