Transcript Digital Security - UC San Diego

Internet Protocol Fundamentals
Gateway to the World
By Eric L. Michelsen
Inductive Logic
Topics
Internet
Point
Protocol
Services
Where in the Stack Is
IP?
IP Addressing
IP Networks and Hosts
IP Network Classes
Multi-homed hosts
Routing
Minimum Host
Configuration
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to Point Links
Subnetting
Classless Inter-Domain
Routing (CIDR)
Private Addressing
DNS
UDP
TCP: Reliable Delivery
IPv6 (IP, the Next
Generation)
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Where in the Stack is IP?
IP is a layer 3 protocol (network layer)
 IP is designed to run over any and all link layers
(layer 2)
 IP folk used to think of a 4-layer stack

7
6
5
4
3
2
1
OSI
Application
Presentation
Session
Transport
Network
Link
Physical
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Telnet, FTP, email, Netware services
IP
UDP, TCP, Novell SPX
IP, IPX, NetBIOS
Ethernet II, IEEE 802.2
10Base-T, T1, V.34, EIA-232
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Application
3
Transport
TCP, UDP
2
Network
IP
1
Physical
3
Internet Protocol Services
 IP v4
(RFC-791, and many others)
 IP provides 3 primary Services:
• Global addressing
• Best-effort (not guaranteed) datagram delivery
• Fragmentation
 Base protocol on which many others are
built
 Upper layers provide reliability as needed
 Fragmentation is inefficient, and generally
avoided.
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IP Addressing
32-bit (4-octet) address, written in dotted decimal:
w.x.y.z
e.g., 206.71.190.4
• w, x, y, and z are octets, ranging from 0 to 255
 Each IP address is globally unique
• except for private addresses
 An IP network is a group of hosts that can
communicate “directly” with each other
• “directly” means no intervening IP devices
 All IP packets include the destination and source IP
address

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IP Networks and Hosts

A typical IP network might be an Ethernet:
Host
Host
Host
Host
206.71.190.1
206.71.190.2
206.71.190.3
206.71.190.4
206.71.190.0
 Each
host interface has an IP address
 An IP address includes two parts: the network
address, and the host address, e.g.
network
206.71.190 .4
host
 All hosts on net have the same network address
 The network as a whole is referred to as host = 0
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Another Sample IP Network
 Full-mesh
Frame Relay network
• Any two hosts can communicate “directly”
 Broadcasts must be duplicated by sender to
each VC
Host
Host
206.71.190.2
PVC
Single IP Interface
206.71.190.1
PVC
PVC
PVC
Host
206.71.190.3
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PVC
PVC
PVC
The whole mesh is
network 206.71.190.0
Host
206.71.190.4
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Classical Class

Network/host address sizes vary in classes:
• Class A: N.h.h.h (0.0.0.0 to 127.0.0.0)
128 networks, 16M hosts per network
 Example:
10.1.1.1
network 10, host .1.1.1

• Class B: N.N.h.h (128.0.0.0 to 191.255.0.0)
16,384 networks, 65k hosts per network
 Example:
132.10.5.17
network 132.10, host .5.17

• Class C: N.N.N.h (192.0.0.0 to 223.255.255.0)
2M networks, 254 hosts per network
 Example:
206.71.190.13 network 206.71.190, host .13

• Classes D & E are “special”
 Host address of all 1s (e.g., 206.71.190.255) means
broadcast to an entire IP network (deprecated)
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Multi-homed Host
 A host
may appear on multiple networks
 Each network interface has an IP address
199.107.10.0
199.107.10.12
multi-homed
Host
206.71.183.4
206.71.183.0
 A multi-homed
host may be used to forward
packets between networks (i.e., as a router)
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Routing
 Connecting
networks into an “internetwork”
Host
Host
Host
Host
192.168.1.0
192.168.20.0
192.168.1.1
192.168.20.1
Router
Router
206.71.183.1
206.71.183.2
206.71.183.0
Host
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Host
Host
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Host
10
Minimum Host Configuration



2 configuration items required for full internetwork access:
• An IP address
• A default router
Host learns new routes from default router with redirects
Every host (not just routers) must maintain a routing table
192.168.1.0
192.168.20.0
192.168.1.1
192.168.20.1
Router
Router
206.71.183.1
forwarded 1st packet
206.71.183.2
1st packet to 192.168.1.x
206.71.183.0
redirect
subsequent packets
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Host
IP 206.71.183.9
Default router
206.71.183.2
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Point-to-Point Links

Numbered Link: standard IP (wasteful)
• All hosts must have same network number
• Wastes a whole network address for 2 hosts
Host
206.71.190.0
206.71.190.1

Host
206.71.190.2
Unnumbered Link: efficient
• No network number
• Host addresses are completely arbitrary
• Used almost exclusively on routers, and host PPP links
Router
unnumbered
206.71.190.3
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Router
199.107.183.15
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Subnet Masks
 The
subnet mask defines which parts of an
IP address are the ‘network’ and ‘host’ parts
 1s in the subnet mask specify network
address bits, 0s specify host address bits
 Standard class subnet masks:
• Class A: 255.0.0.0
•
•
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11111111.00000000.00000000.00000000
Class B: 255.255.0.0
11111111.11111111.00000000.00000000
Class C: 255.255.255.0
11111111.11111111.11111111.00000000
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Subnetting

Creates networks smaller than the default for their
class (breaks up Class A, B, & C networks)
• Example: subnet mask 255.255.255.192 =
•
11111111.11111111.11111111.11000000
creates a subnet of 64 addresses (62 hosts)
Can use 255.255.255.0 on an (otherwise) Class B
network to create 256 Class-C-size subnets (254 hosts)
Network part is always on left end of subnet mask
 Handy table: 128
1000 0000
240
1111 0000

192
224

1100 0000
1110 0000
248
252
1111 1000
1111 1100
Sometimes written as /n, where n is # bits in
Network part, e.g., /26 => 255.255.255.192
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Examples of IP Subnetting
192.168.1.0

192.168.1.0/24 (mask 255.255.255.0)
• standard Class C
• 254 hosts: 192.168.1.1 - 192.168.1.254



192.168.2.0/25 (mask 255.255.255.128)
• 126 hosts: 192.168.2.1 - 192.168.2.126
192.168.2.128/26 (mask 255.255.255.192)
• 62 hosts: 192.168.2.129 - 192.168.2.190
192.168.2.192/27 (mask 255.255.255.224)
• 30 hosts: 192.168.2.193 - 192.168.2.222
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192.168.1.0/24
192.168.1.255
192.168.2.0
192.168.2.0/25
192.168.2.127
192.168.2.128
192.168.2.128/26
192.168.2.191
192.168.2.192/27
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CIDR
 Classless
Inter-Domain Routing
 Eliminates Class A, B, and C networks.
 Subnet masks must be specified for
everything
• This is a 3rd piece of configuration now
required by an IP host:
IP address
 Subnet mask
 Default Router

 Widely
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used, and growing
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Private Addresses
 IETF
set aside some addresses for “private”
use:
• 1 Class A network
10.0.0.0
• 16 Class B networks 172.16.0.0 - 172.31.0.0
• 256 Class C networks 192.168.*.0
 Internet
routers are configured to discard
packets addressed to these addresses
 These addresses are not visible to the
Internet, so multiple sites can use them at
will
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DNS: Domain Name System
RFCs 1034, 1035
 Memorizing IP addresses is difficult
 DNS is a distributed directory of names, and
associated IP addresses, and other info
• “First DNS server” is a 4th piece of IP host config
 Hierarchical system of shared authority
• Right parts are higher authority than left

www.enterprise.com
Enterprise
InterNIC
Administered Administered
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UDP: User Datagram Protocol
RFC 768
 Built above IP (Layer 4, Transport)
 Best-effort, datagram (packet) delivery
(connectionless)
 Adds an additional addressing layer: port
• Each UDP datagram includes a 16-bit destination and

•
16-bit source port
There are many “well-known” ports, which essentially
act as Server IDs or Protocol IDs for UDP
DNS
 BOOTP/DHCP
 TFTP
 SNMP

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port 53
ports 67 (server), 68 (client)
port 69
port 161
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TCP: Transmission Control Protocol





RFC 793, plus many modifications
Reliable, error-corrected stream of data
Connection oriented (has setup and teardown)
Uses a highly efficient, self-adjusting pacing mechanism
for high throughput
No packetization (or frame) boundaries
• Packetization of data stream into IP packets is invisible to the
application layer


Packet boundaries (if needed) must be created by higher
layers
Like UDP, has ports. Well known ports:



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FTP control
Telnet
SMTP
port 20
port 23
port 25
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IPv6 (IPng)
Primarily intended to address the problem of
running out of IP addresses
 Aka Network Engineer Employment Act of 1994
• Nearly every IP protocol must change
• Nearly every IP software application must change
 Addresses extended to 16 octets (128 bits)
• Enough for each molecule on the surface of the earth to

have its own IP address
Part of address is locally assigned
 Fragmentation confined to endpoints (routers don’t
fragment, hosts do)

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
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
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