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Introduction to LAN/WAN
Network Layer (part II)
Topics
The Network Layer
IP, IP addresses
ARP (5.5.4)
OSPF (5.5.5)
BGP (5.5.6)
Internet Structure
)
Backbones: high bandwidth lines, fast routers
)
Regional networks attached to backbones
)
LANs (universities, companies, ISP, etc)connected to regional network
Internet Protocol (IP)
)
IP concerned with routing (best effort)
)
Interesting options:
- security, strict source routing, loose source routing,
record route, timestamp
IP Addresses
)
IPv4: 32 bit addresses: 2
32
addresses (e.g.
ccc4.wpi.edu
has IP address 130.215.36.158)
)
Addresses controlled by ICANN
)
Previously
classfull
addressing (A, B, C, etc)
- Class A: fix first bit at 0, 2
7
networks, 2
24
hosts
- Class B: start with 10, 2
14
networks, 2
16
hosts, etc
- Class C: start with 110, 2
29
networks, 2
8
hosts
IP Addresses
)
Disproportional demands for address classes:
- Few organizations have up to 2
24
hosts (class A)
16
hosts)
- Quick depletion of some classes (class B) while others
remained un-used (class A)
)
Now Classless InterDomain Routing (CIDR)pronounced “
cydar”
- Basically allow variable network address (subnet mask)
and host address part
- Indicate length by adding subnet mask– Example: 223.1.1.0/24 means first 24 bits should be
treated as network address (subnet mask)
)
CIDR was temporary solution => IPv6 (128 bits)
Network to Data Link Address
Translation
Internet hosts use IP
Data link layer does not understand IP
- Ethernet uses 48-bit address– ex: ifconfig gives
00:10:4B:9E:B3:E
Q: How do IP addresses get mapped ontodata link layer addresses, such as Ethernet?
A: The Address Resolution Protocol (ARP)
Example 1
Host 1 wants to sends message to Host 2, say “
mary@eagle.cs.uni.edu
Docsity.com
Address Resolutioning
Host 1 broadcasts packet on LAN with IPaddress
asking “Who owns
IP address
Each machine checks its IP address.
Host 2 responds w/Ethernet address (E2)
Address Resolution Protocol
(ARP)
Host 1 data-link can then encapsulate IPpacket in frame addressed to E2 and dump
Ethernet board on Host 2 recognizes, stripsframe header and sends up to IP layer
ARP Optimizations
)
Send to H2 again?
- cache requests (time out in case of new card)
)
Many times, H1 requires ack from H
, E1)
- H2 caches and uses if needed
)
Hosts broadcast mapping when boot
- host looks for its own IP address
should get no answer, else don’t boot
- other Ethernet hosts all cache answer
Solutions
Solution 1
- CS router configured to respond to ARP
requests for
- Host 1 makes an ARP cache entry of
, E3)
sends all traffic to Host 4 to CS router
Proxy ARP
Solution 2
- Host 1 knows Host 4 is on different subnet
sends to CS router
- CS router doesn’t need to know about remote
networks
Either way ...
Host 1 packs IP into Enet frame to E
CS router receives frame, removes packet
to
Sends ARP packet onto FDDI
is at F
Puts packet into payload of FDDI frame andput on ring
EE router receives frame, removes packet ...
Routing on the Internet
Internet made up of Autonomous Systems(AS)
Standard for routing inside AS
- Interior Gateway Protocol–
OSPF
Standard for routing outside AS
- Exterior Gateway Protocol–
BGP
ASes, Backbones and Areas