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IESG Approval or Standards Action
The Internet Control Message Protocol (ICMP) has many messages that are identified by a "type" field.
This is a complete rip from http://wiki.xtronics.com/index.php/IP_Subnet_Masks
IP mask numbers are used to divide internet addresses into blocks called subnets. The mask number represents the number of 1s in the binary of the address that is 'masked" against the address so that it ignores the last bits which are for the group of IP addresses in the masked address.
The first address of a subnet block (all 0s) is called the network address or network ID. The last address (all 1s) is the broadcast address of the network. Typically the network address +1 or the broadcast address -1 is the gateway to the internet. The 'slash' notation (ie /24) is known as CIDR format, while the more conventional 255.255.255.0 notation is considered a subnet mask.
Many times when working with a client network or working on our own we have the need to test, document and validate certain networks configurations in a test environment. Sadly not many have the money to have one so as to test different scenarios so as to gage the impact that this changes might have on the production network. For a majority of configuration when it comes to system settings and routing a virtualized environment can be of great help, sadly anything ASIC or HW Specific configurations. On this wiki page I will cover how to virtualize JunOS operating system to aide with testing and validating. I did this for a friend who needed to migrate the configuration of several of his Juniper Routers to a newer version of the OS and Hardware and also asked me for recommendations for hardening the routers. I do have to say I really like JunOS specially since it is a full FreeBSD subsystem underneath to wish a user has access to.
Most of this information is a rip from Other Sites so keep that in mind as you read through this.