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Subnet Mask Addressing
Assume a configuration as follows…
SOURCE ADDR = 128.100.0.1
DESTINATION ADDR = 128.100.69.100
SUBNET MASK = 255.255.255.0
BROADCAST MASK = 128.100.0.255
In this case, the DESTINATION PACKET will go to the DEFAULT GATEWAY ADDRESS
Assume a configuration as follows…
SOURCE ADDR = 128.100.0.1
DESTINATION ADDR = 128.100.69.100
SUBNET MASK = 255.255.0.0
BROADCAST MASK = 128.100.255.255
In this case, the DESTINATION PACKET will NOT go to the DEFAULT GATEWAY ADDRESS and will be routed within the local network.
To determine via the SUBNET MASK if a destination packet goes to the GATEWAY node or not, follow these simple rules:
- Perform a LOGICAL AND of the destination address of the mask of the local subnet address
- Perform an EXCLUSIVE OR on the result of the previous operation and the local net address of the local interface (eg., source address)
- If the result of the previous operation is a ZERO, the destination is assummed to be reachable directly through one of the local interfaces ELSE the packet is forwarded to the DEFAULT GATEWAY ADDRESS.
$HOME/.netrc
Automatic login information for the ftp and rexec commands.
syntax: machine <hostname> login <username> password <password>
If this file contains a password entry (optional), the file permissions must be set to 600 (rw for owner only) or else the error message rshd: 0826-813 Permission is denied is generated.
The file .netrc must exist on the client that your executing the ftp or rexec command from and the <hostname> must specify the name of server to connect to.
For FTP only, you can initiate file transfers via a macro definition (up to 16 macros can be defined). This file resides on the client machine.
An example of the contents of a .netrc file…
machine sys8 login transfer password now macdef init
get /tmp/database.log /tmp/log/database.log
quit
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$HOME/.rhosts
Defines which client users are not required to supply a login password for the rcp, rlogin, or rsh commands using a user account on the server. The file resides on the server machine under the home directory of the user account being logged into. An ALIAS entry cannot be specified.
syntax: <hostname> <username>
/etc/hosts.equiv
Defines which clients are permitted to execute commands by the lpd, rlogind, rcpd, or rshd daemons without supplying a password.
syntax: <hostname>
/etc/hosts.lpd
Defines which clients are permitted to print jobs to the print server.
arp – Address Resolution Protocol
Displays the translation table between TCP/IP addresses and network addresses. This physical address is six bytes long and is unique for every network board. The first three bytes of the address represent a vendor’s ID. The last three bytes are assigned by the manufacturer.
Some examples…
NOVELL 00 00 1B NOVELL 08 00 14 3COM 02 60 8C SUN 08 00 20 IBM 08 00 5a DEC AA 00 04
Note that an IP address is really assigned to a network interface and not a host. A host may have more than 1 network interface as illustrated by the output of the netstat -i command.
Useful tool for debugging network problems.
- For example, if you are using the DOS SERVER daemons to connect PCs to the RS/6000 and connections aren’t being made, check the arp table to see if there is a hostname entry for the name of the PC you are connecting from. If there is an entry in the arp table, then you know that packets are getting across and the problem lies with the DOS SERVER or PC DOS CLIENT software.
- Further, you may have configured two or more hosts with the same internet address. If this is the case, its possbile that one connection may get hung. That’s because the entry in your arp table gets overwritten by the new connection. View the arp table and you’ll find that the internet address may now be bound to a new physical network address. The connection that is hung, will not show the correct physical address for that IP address. It shows the new physical address of the machine that has the same IP address of the previous connection.
- Another reason to use this utility for a debugging tool is to show what network adapter card address is mapped to an internet address. If by chance, you have two nodes on your network with the same internet address, you will have periodic problems keeping a connection to one of those two machines until a unique internet address is assigned to each machine.
Examples: arp -a (Displays all addresses that have been translated by the address resolution
protocol.)
Outputs: dospc (193.0.0.8) at 10:0:5a:2b:69:64 [token ring]
eai1 (192.0.0.3) at (incomplete)
An incomplete entry in the network address field means the node in question could not be contacted. Probably reasons are:
- node doesn’t exist on the network
- node isn’t connected to network
- network is not terminated properly
- problem with network cabling
- software on destination host has a bug
arp -d dospc (Remove the dospc entry from the arp table)
finger
Lists the current users logged in from remote machines, including username, hostname, idle time, and so on
Example: finger -i (alias f -i)
host
This command returns the internet address when a hostname is specified or the hostname when the internet address is specified.
| Examples | What it does |
| host www.ahinc.com | www.ahinc.com 209.218.236.72 |
| host 209.218.236.72 | www.ahinc.com 209.218.236.72 |
hostname – Sets or displays the name of the host system
| Examples | What it does |
| hostname | Displays the current hostname value |
| hostname rs520 | Sets the name of the machine to rs520. Make ure that you set the system name to rs520 by executing the uname command |
uname – Sets or displays the system name
| Examples | What it does |
| uname -S rs520 | Sets the system name |
| uname -a | List the system name |
ifconfig – Configures or displays the network interface parameters for a tcp/ip network
| Examples | What it does |
| ifconfig en0 | Indicates if the en0 interface is up and running. en0: flags=2000063<UP,BROADCAST,NOTRAILERS,RUNNING,NOECHO> inet 192.0.0.1 netmask 0xffffff00 broadcast 192.0.0.255 |
| ifconfig en0 up | Before this command is executed the state of the en0 interface is down (doesn’t show UP): en0: flags=2000062<BROADCAST,NOTRAILERS,RUNNING,NOECHO> inet 192.0.0.1 netmask 0xffffff00 broadcast 192.0.0.255After this command is executed the state of the en0 interface is: en0: flags=2000063<UP,BROADCAST,NOTRAILERS,RUNNING,NOECHO> inet 192.0.0.1 netmask 0xffffff00 broadcast 192.0.0.255 |
| 1) ifconfig en0 down2) ifconfig en0 detach | Adapter statistics for the ethernet card get reset to zero. See the -v display for further information. |
iptrace – Debug trace utility for IP packets
| Examples | What it does |
| iptrace -a -s 193.0.0.1 -i en0 network.log | All TCP/IP packets that originate from host address of 193.0.0.1 with a ethernet network interface (en0) will be logged to the file network.log. |
ipreport – Generates a packet trace report generated from the iptrace facility
Example: ipreport network.log
Dumps the packets that were trapped from the iptrace session in ascii format for inspection. Must kill the iptrace process before this report can be displayed.
nameserver (resolv.conf)
| Examples | What it does |
| host ahinc or arp -a |
If the arp -a or host commands hang, usually means the name server daemon (/etc/named) is not running. Use the command ps -ef|grep named to check if named is running. |
| Telnet login responce is very slow. | Make sure the /etc/resolv.conf file has a valid entry in it and the /etc/hosts file has the appropriate entries for all known client nodes.Contents of an example of a /etc/resolv.conf: nameserver 192.168.1.1 domain ahinc.comWhen using the named process, each node should have at least the loopback and local host name defined in the /etc/hosts file.top of page |
netstat – Shows network statistics
| Examples | What it does |
| netstat -f inet | List statistics of AF_INET address family for remote communications |
| netstat -f unix | List statistics of AF_UNIX address family for local communications. |
| netstat -m | Shows current network interfaces that are up and running.
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| netstat -r | If the non-loopback entry has a G in Flags field (g=gateway), then the ping command may hang. Change the Flags field to U only. Make sure the ROUTED daemon is running. To start: startsrc -s routed. |
| netstat -nr | Route Tree for Protocol Family 2: |
| netstat -sr netstat -s |
Displays statistics for each protocol (ip, icmp, tcp, udp) |
| netstat -v | Shows statistics about network interface cards installed.
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| netstat -v | chdev -l ent0 -a receive_size=60 |
nfsstat
Lists statistics about network file system (NFS) and remote procedure calls (RPC).
Also distinguishes between client and server information.
no – Configures network options.
The no command ONLY operates on the currently running kernel. This command must be run each time the system is rebooted.
| Examples | What it does |
| no -o tcp_ttl=40 | Specifies the time to live for TCP packets (40 ticks). |
| no -o thewall=3000 | Sets the size of the mbuf pool to 3 MB (default is 2 MB). If requests for mbufs has been denied, this should prevent requests from being denied. |
| no -a ipforwarding=0 | Turns off IP forwarding |
| no -o tcp_sendspace=8192 no -o tcp_recvspace=8192 |
Increasing the buffers to 8k, performance gains of up to 10-20% have been attained. |
ping
Sends an ICMP ECHO_REQUEST message to a network host every second.
| Examples | What it does |
| ping rs520 | Quick way to determine if the network host you wish to communicate with is either up and running or is configured properly. If ping intermediately reports back packets (5 packets sent, 1 is lost, 2 packets sent, 1 lost, etc), suspect a bad connection. |
| ping 192.0.0.255 | Generates responses from all the other nodes connected on a class C network. If you wish to see what nodes are up and responding on your network, this is a simple test to run. |
spray
Sends a specified number of packets over the network and reports back performance statistics.
| Examples | What it does |
| /usr/etc/spray AIX -c 1000 -l 1400 -d 1 | Send 1000 packets to the host named AIX with a delay of 1 micro-second, where each packet sent is 1400 bytes. This uses the RPC protocol. |