OSI Layers Fundamentals

OSI Layers:
Transport layer:
1. Two main protocols (TCP & UDP).
TCP - Requires handshake and responses. Reliable.
UDP - Not reliable.
In a streaming environment, TCP would start of the session to be continued with UDP.

2. HASH value in torrent file.
Parity bits error detection and correction.

3. If you can ping the gateway, then Network and Data Link layer are working.

4. Session layer - if you know web programming, you should been familiar with session.
step 1. Physical layer - Check cable, ethernet card blinking.
step 2. Ping loopback (127.x.x.x) - Checking the data link layer.
step 3. Ping DNS server, Ping gateway -

5. nslookup
C:\Documents and Settings\admin>nslookup
*** Can't find server name for address Non-existent domain
*** Default servers are not available
Default Server: UnKnown
> set q=all
> it-sideways.com
Server: UnKnown
Non-authoritative answer:
it-sideways.com internet address =
it-sideways.com nameserver = park10.secureserver.net
it-sideways.com nameserver = park9.secureserver.net
primary name server = park9.secureserver.net
responsible mail addr = dns.jomax.net
serial = 2005062500
refresh = 28800 (8 hours)
retry = 7200 (2 hours)
expire = 604800 (7 days)
default TTL = 86400 (1 day)
it-sideways.com MX preference = 10, mail exchanger = mailstore1.secureserver.net

park10.secureserver.net internet address =
mailstore1.secureserver.net internet address =
OSI layers in details:

6. RPC (session layer) - Blaster worm attack using RPC protocol. Check out the dependencies using services.msc

7. Transport layer - 3 ways handshake.

8. Network layer - Route update packet (Router advertisement)

9. Data Link - Buffer management.
MAC address - First Three hexa identifies vendors (suppliers). This is known as the vendor code.

10. Physical Link - Signal and media.
Ethernet Networking:
1. CSMA/CD (half-duplex) is outdated.
2. Hub - generally half-duplex only.
3. Switch - Full-duplex (can be used for half-duplex)
4. Ethernet (Frame) types:
802.2 (Outdated)
SNAP (Outdated)

Cisco Three-layer model:
Design network - Rule of thumb.
1. 80% of the traffic should stay on the same network.
Unicast - one to one
multicast - one to many
broadcast - one to all (shouting)

Core layer:
1. Transfer data as fast as possible.
usually there would be two multi-layer switches for failover (backup).
2. Fault tolerance - One core switch down, you have the ability to bring it up.
Dedundancies - On standby. (RAID - hotswap)

Distribution layer:
1. Routing (inclusive of VLAN)
2. NAT
3. Redistributing of protocols (communications between protocols)

Access Layer:
1. Segmentation
Besides the OSI and Cisco three layer model,there is another model called "Avid" model.

1. CAT-5 (RJ 45) maximum stretch = 100 meters. (more than that will need a repeater)
2. Core layer would require Gigabit Ethernet.
3. Serial point-to-point connection.
232 cable.
DCE (Data channel equipment) - connect to ISP.
- Assign clock rate (real wire speed)
- Determines the real wire speed
DTE (Data terminal equipment)
- Assign bandwidth (not real bandwidth)


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