Network Layer

1
Network Layer

• IS250
• Spring 2010
• chuang_at_ischool.berkeley.edu

2
Network Layer

• Application (layer 7) specific to application
  need
• Transport (layer 4) end-to-end delivery,
  congestion and flow control
• Network (layer 3) addressing, routing
• Data Link (layer 2) framing, error detection
• Physical (layer 1) bits (0/1), voltages,
  frequencies, wires, pins,

3
L3 Outline

• Addressing
• Internet Protocol (IP) address notation address
  allocation address translation
• Packet switching
• Routing BGP, OSPF, RIP
• Packet forwarding IP fragmentation, TTL,
• Extensions
• IP Multicast QoS Mobile IP IPSec
• Evolution
• IPv6

4
Internet Protocol

• Supports two main functions
• Addressing
• Packet switching (routing)
• Allows packets to traverse multiple networks
• hence the term inter-networking
• Delivers packet to specified destination host
• Best effort service model
• deliver as reliably and as soon as it can

5
IP Does Not

• Guarantee latency for packets that are delivered
• Guarantee delivery, or notify source host if
  packet is not delivered
• Guarantee order of delivery
• Guarantee integrity of packet payload
• Maintain conversational context (each packet is
  independent)
• Specify process that should receive the packet at
  destination host

6
Definition of the Internet

• "Internet" refers to the global information
  system that --
• (i) is logically linked together by a globally
  unique address space based on the Internet
  Protocol (IP) or its subsequent
  extensions/follow-ons
• (ii) is able to support communications using the
  Transmission Control Protocol/Internet Protocol
  (TCP/IP) suite or its subsequent
  extensions/follow-ons, and/or other IP-compatible
  protocols and
• (iii) provides, uses or makes accessible, either
  publicly or privately, high level services
  layered on the communications and related
  infrastructure described herein.
• Resolution passed by the Federal Networking
  Council (FNC) October 24, 1995

7
IP Address

• An IP address identifies a network interface,
  i.e., a connection between a computer and a
  network, not a specific computer.
• A computer with multiple network interfaces
  (e.g., a router) must be assigned one IP address
  for each interface.
• IP (version 4) addresses are 32 bits long
• 232 4,294,967,296 unique IPv4 addresses
• IPv6 addresses are 128 bits long
• 2128 340,282,366,920,938,463,463,374,607,431,768
  ,211,455 3.41038 unique IPv6 addresses

8
Dotted Decimal Notation

• Represent each byte (8 bits) in decimal separated
  by dots
• 128 32 226 87
• Hostname www.ischool.berkeley.edu
• IP address 128.32.226.87

0
16
31
8
24
1 0 0 0 0 0 0 0
0 0 1 0 0 0 0 0
1 1 1 0 0 0 1 0
0 1 0 1 0 1 1 1
Domain Name Service (DNS) performs translation
9
Example of Dotted Decimal Notation

• Four decimal values per 32-bit address
• Each decimal number
• Represents eight bits
• Is between 0 and 255

10
IP Address

• Divided into two parts
• Prefix identifies network
• Suffix identifies host
• Global authority (IANA) assigns unique prefix to
  network
• Local administrator assigns unique suffix to host

11
Illustration of Router Addresses

• Routers usually have multiple IP addresses
• One address needed for each network interface
• Address prefix identifies network

12
Special Addresses (Reserved)
13
Original Classes of Addresses

• Initial bits determine class
• Class determines boundary between prefix and
  suffix

14
IP Addresses
15
Classful Addresses and Network Sizes

• Maximum network size determined by class of
  address
• Class A large (mostly assigned or reserved)
• Class B medium (mostly assigned)
• Class C small

16
IP Address Exhaustion

• 32 bit address space not enough
• Exacerbated by inefficient allocation of
  addresses
• Several approaches to deal with problem
• Increase IP address length (IPv6)
• Overcome inefficient address allocation
• Subnetting
• Classless inter-domain routing (CIDR)
• Allow sharing of addresses
• Network Address Translation (NAT)
• Dynamic Address Allocation (DHCP)

17
Subnetting

• Problem
• Class A and class B address blocks have too many
  host IDs allocated (tremendous waste of
  addresses)
• Can we support multiple physical networks
  (subnets) within a single class A or class B
  address block?
• Solution
• External routing based on Network ID
• Internal routing based on Subnet ID
• Significantly reduces the number of entries
  required in Internet routing tables.

18
Subnetting

• Host ID portion is divided into subnet ID and
  host ID
• Routers and hosts use a subnet mask to separate
  the subnet id from the host id.
• Example supporting 256 subnets within a class B
  network

IP address (Class B)
16
8
8
Network ID
Subnet ID
Host ID
IP address (Class B)
24
8
111111111111111111111111
00000000
Subnet mask (255.255.255.0)
19
Example Router Operation with Subnet Mask

• Router R
• Receives incoming IP packet (128.32.226.87)
• Applies subnet mask (255.255.255.0) via logical
  AND operation
• Gets result (128.32.226.0)
• Also applies subnet mask to its own addresses
  (one on each subnet)
• Gets 128.32.1.0, 128.32.226.0
• There is a match (128.32.226.0)
• Router delivers to host on that network

Internet
128.32.0.1
R
128.32.226.1
128.32.1.1
128.32.1.0
128.32.226.0
128.32.1.87
128.32.226.87
128.32.0.0
20
Classless Inter-Domain Routing (CIDR)

• Drops notion of fixed classes
• Represent network address as ltaddress/prefix_size
  gt
• E.g., 65.0.0.0/8 or simply 65/8 192.1.2.0/22
• Prefix_size is length of network id field (in
  bits)
• CIDR allows arbitrary prefix size
• Each network can be as large or small as needed
  (power of two)
• Backward-compatible with network classes
• Class A networks have prefix size of 8
• Class B networks have prefix size 16
• Class C networks have prefix size 24

21
CIDR Example

• Combining four class C networks
• 192.1.4.0/24
• 192.1.5.0/24
• 192.1.6.0/24
• 192.1.7.0/24
• First two can be combined as 192.1.4.0/23
• Last two can be combined as 192.1.6.0/23
• All four can be combined as 192.1.4.0/22

22
Classless Addressing
(drawings not to scale)
Class-based
A
B
C
D
0
232-1
128.9.0.0
Classless
65/8
128.9/16
0
232-1
216
128.9.16.14
Most specific route longest matching prefix
23
Network Address Translation

• Network Address Translators (NATs) allow multiple
  hosts within a local network to share a single IP
  address
• From outside perspective, the network appears as
  a single end host
• Can use arbitrary IP address scheme within
  network
• Typically 10.0.0.0 or 192.168.0.0
• Most common implementation is actually Network
  Address and Port Translation (NAPT)
• Maps internal ltaddress, portgt to external
  ltaddress, portgt where ports are transport layer
  (Layer 4) addresses
• Incorporated into most residential gateway
  routers today
• Controversial because it violates layering
  principle

24
NAT Example Outbound
Source David Maltz
25
NAT Example Inbound
Source David Maltz