CS469/569 - Linux and Unix Administration and Networking

Spring 2022

Internet Protocol (IP) (man 7 ip)

Provides a route-able protocol for delivering data-gram packets across networks based on IP address. There are two version, version 4 (IPv4) which uses 32 bit addresses (< 4 billion addresses) and a less widely used version 6 (IPv6) which uses 128 bit addresses (< ~340 undecillion addresses). For now IPv4 still remains the primary Internet protocol:

An IPv4 Header contains among other information:
- Source IP address
- Destination IP address
- Total length of the IP packet
- Protocol - i.e. TCP / UDP - The protocol encapsulated within the IP packet
- Time to live (TTL) - A number decremented at every hop along the packets journey, upon reaching zero, the packet is discarded. This prevents a packet caught in a loop from being passed around forever.
- Other fields for IP fragmentation and options.
Packets are routed based on destination. It isn't necessarily the case that a packet will be returned along the same path that one was sent. It is also the case that the source address can be completely bogus.

Files:

/etc/protocols

List of protocols (man 5 protocols) (mostly useful to programmers)

Programs:

> ifconfig

Configure a network interface (being deprecated)

Example:

ifconfig eth0 139.102.14.201 netmask 255.255.255.0 broadcast 139.102.14.255
or maybe: ifconfig eth0 139.102.14.201/24

Configures eth0 for:

IP address 139.102.14.201

Netmask 255.255.255.0

Broadcast address 139.102.14.255

> ip

show/manipulate routing, devices, policy routing and tunnels. Too many options to list. Typically not used by humans, but in scripts.

IPv4 Addresses:

Consist of 32 bits, typically listed in 4 8-bit "octet" dot-decimal notation.
Distributed by the Internet Assigned Numbers Association (IANA) a department of ICANN (Internet Corporation of Assigned Names and Numbers):
- Delegates allocations to Regional Internet Registries (RIRs) which each address different areas of the world.
- Administers the data in the root domain name servers
- ICANN also:
  - Administers protocol parameters of IETF (Internet Engineering Task Force (who publish the RFCs))
  - Maintain a time zone database.
IPv4 Private addresses:
- These are addresses that are not routed on the internet (but might be routed on the local network.)
- They consist of three reserved blocks:

CIDR Starting address Ending address Number of IPs

10.0.0.0/8 10.0.0.0 10.255.255.255 (~16M)

172.16.0.0/12 172.16.0.0 172.31.255.255 (~1M)

192.168.0.0/16 192.168.0.0 192.168.255.255 (~65K)

The 127.0.0.0/8 block is the loopback or localhost address range and is a virtual network range that exists only with a machine itself via the virtual loopback device lo or lo0, typically assigned the address 127.0.0.1.
IP addresses are usually assigned in two ways:
- Static assignment, where the IP address is configured manually on the host and does not change.
- Dynamic assignment, where a protocol, such as DHCP (Dynamic Host Configuration Protocol) or BOOTP is used to request an IP address.
The Broadcast address for a host is the host part of the IP address where all bits are 1's.
(139.102.14.201&255.255.255.0)|0.0.0.255 -> 139.102.14.255
Packets with the destination set to the broadcast address are typically recieved by all machines on the local network.
Multicast IP addresses: 224.0.0.0/4 (224.0.0.0-239.255.255.255)

Network commands:

> route

print and set routes

> routel

List routes

> traceroute [-n] hostname

Show routes taken from host to destination.

# echo "1" > /proc/sys/net/ipv4/ip_forward

Enables packet forwarding between interfaces. Lets Linux be a gateway.

> ping hostname

Send ICMP echo's to a machine.

Subnetting:

Classful network design:
- Originally divided networks into 3 sizes, Class A (24 bits), Class B (16 bits) (ISU's network was originally a Class B network (139.102.0.0) and Class C (8 bits).
Classless Inter-Domain Routing (CIDR)
- Prior to CIDR IPv4 networks were represented using dot-decimal notation for both IP address and netmask: 192.168.100.0/255.255.255.0 (classful) == 192.168.100.0/24 (CIDR)
CIDR divides an IP network into smaller logical sub-networks. Given:

[ Network prefix ][ Host number ]
                      /       \
[ Network prefix ][Subnet #][Host #]

The host part is determined by (IP address & ~ Netmask)
All zeros in the host part is reserved for the network ID.
All ones in the host part is reserved for the broadcast address.

Examples:

192.168.1.0/30: (2 bits for host part)

11111111.11111111.11111111.111111 00 Netmask: 255.255.255.252

11000000.10101000.00000001.000000 00 Network address: 192.168.1.0

11000000.10101000.00000001.000000 01 1st IP: 192.168.1.1

11000000.10101000.00000001.000000 10 2nd IP: 192.168.1.2

11000000.10101000.00000001.000000 11 Broadcast address: 192.168.1.3

192.168.1.4/30: (2 bits for host part, second network)

11111111.11111111.11111111.111111 00 Netmask: 255.255.255.252

11000000.10101000.00000001.000001 00 Network address: 192.168.1.4

11000000.10101000.00000001.000001 01 1st IP: 192.168.1.5

11000000.10101000.00000001.000001 10 2nd IP: 192.168.1.6

11000000.10101000.00000001.000001 11 Broadcast address: 192.168.1.7

192.168.1.10/16: (16 bits for host part)

11111111.11111111. 00000000.00000000 Netmask: 255.255.0.0

11000000.10101000. 00000000.00000000 Network address: 192.168.0.0

11000000.10101000. 00000000.00000001 1st IP: 192.168.0.1

11000000.10101000. 11111111.11111110 last IP: 192.168.255.254

11000000.10101000. 11111111.11111111 Broadcast address: 192.168.255.255

139.102.14.201/24: (8 bits for host part):

11111111.11111111.11111111. 00000000 Netmask: 255.255.255.0

10001011.01100110.00001110. 00000000 Network address: 139.102.14.0

10001011.01100110.00001110. 00000001 1st IP: 139.102.14.1

10001011.01100110.00001110. 11111110 last IP: 139.102.14.254

10001011.01100110.00001110. 11111111 Broadcast address: 139.102.14.255