24.7 Networking Fundamentals

24.7.1 Objective 1: Fundamentals of TCP/IP

24.7.1.1 Addressing and masks

• An address mask separates the network portion from the host portion of the 32-bit IP address.

• Class A addresses have 8 bits of network address and 24 bits of host address.

• Class B addresses have 16 bits of network address and 16 bits of host address.

• Class C addresses have 24 bits of network address and 8 bits of host address.

• Subnets can be defined using the defined "class" schemes or using a locally defined split of network/host bits.

• The all-zero and all-ones addresses are reserved on all subnets for the network and broadcast addresses, respectively. This implies that the maximum number of hosts on a network with n bits in the host portion of the address is 2^n-2. For example, a Class C network has 8 bits in the host portion. Therefore, it can have a maximum of 2^8-2=254 hosts.

24.7.1.2 Protocols

TCP/IP is a name representing a larger suite of network protocols. Some network protocols maintain a constant connection while others do not.

IP

The Internet Protocolis the fundamental building block of the Internet. It is used by other protocols.

ICMP

This connectionless messaging protocol uses IP. It is used for flow control, detection of unreachable destinations, redirecting routes, and checking remote hosts (the ping utility).

UDP

The User Datagram Protocol is a connectionless transport agent. It is used by applications such as DNS and NFS.

TCP

The Tranmission Control Protocol is a connection-oriented transport agent. It is used by applications such as FTP and Telnet.

PPP

The Point-to-Point Protocol is used over serial lines, including modems.

24.7.1.3 TCP/IP services

• Inbound network requests to a host include a port number. Common port numbers are listed in Table 24-4.

Table 24-4. Common Port Assignments

Port Number	Assigned Use	Description
20	FTP data	When an FTP session is opened, the binary or ASCII data flow to the server is conducted using port 20, while control information flows on port 21. During use, both ports are managed by an ftp daemon, such as wu-ftpd or PROftpd.
21	FTP control
23	Telnet server	Inbound Telnet requests are sent to server port 23 and processed by telnetd.
25	SMTP server	This port is used by mail transfer agents (MTAs), such as sendmail.
53	DNS server	This port is used by the Domain Name System server, named.
67	BOOTP/DHCP server	This port is used by BOOTP or the more commonly used DHCP server.
68	BOOTP/DHCP client	This port is used by the client side for BOOTP/DHCP.
80	HTTP server	Web servers, such as Apache (httpd), usually listen in on this port.
110	POP3	The Post Office Protocol (POP) is used by mail client programs to transfer mail from a server.
119	NNTP server	This port is used by news servers for USENET news.
139	NetBIOS	This port is reserved for Microsoft's LAN network manager.
143	IMAP	An alternate to POP3, IMAP is another type of mail server.
161	SNMP	Agents running on monitored systems use this port for access to the Simple Network Management Protocol.

• Ports are assigned to specific programs. Definitions are stored in /etc/services.

• Ports 1-1023 are privileged ports, owned by superuser processes.

24.7.1.4 TCP/IP utilities

• ftp implements the File Transfer Protocol client for the exchange of files to and from remote hosts.

• The telnet client program implements a Telnet session to a remote host.

• ping sends ICMP echo requests to a remote host to verify functionality.

• dig obtains information from DNS servers.

• traceroute attempts to display the route over which packets must travel to a remote host.

• fwhois queries a whois database to determine the owner of a domain or IP address.

24.7.2 Objective 3: TCP/IP Troubleshooting and Configuration

24.7.2.1 Network interfaces

• Interfaces are configured through a number of configuration files.

• /etc/hostname contains the assigned hostname for the system.

• /etc/hosts contains static mappings between IP addresses and names.

• /etc/nsswitch.conf directs system library functions to specific name server methods such as local files, DNS, and NIS.

• /etc/host.conf controls name resolution for older libraries.

• /etc/host.conf is only rarely used and is replaced by /etc/nsswitch.conf.

• /etc/resolv.conf contains information to direct the resolver to DNS servers.

• /etc/networks sets up equivalence between addresses and names for entire networks.

• The host command returns DNS information.

• The hostname, domainname, and dnsdomainname commands set or display the current host, domain, or node name.

• The ifconfig command configures network interfaces. It is used to create and configure interface parameters, usually at boot time. Parameters include the IP address and subnet mask.

• The netstat command displays network connections, routing tables, interface statistics, masquerade connections, and multicast memberships.

• The route command displays the routing table and can add or delete routes from the table.

24.7.2.2 DHCP

• DHCP is the Dynamic Host Configuration Protocol. It is used to assign an IP address and other information to a client system.

• The DHCP server is dhcpd.

• A DHCP server offers an address for a finite amount of time known as a lease.

24.7.3 Objective 4: Configure and Use PPP

• PPP is used to make a network connection over a serial interface. This could be a direct cable or modem connection.

• PPP is a peer protocol; there are no clients or servers.

• pppd is the PPP daemon, called when a PPP interface is needed. It uses a chat script to send configuration commands to a modem prior to dialing.

• Basic authentication for PPP can be done in clear text via the chat script. However, the PAP, CHAP, and MSCHAP methods encode their authentication information into the PPP data stream.