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Full Discussion: Subnetting
Special Forums IP Networking Subnetting Post 302993570 by drysdalk on Saturday 11th of March 2017 07:41:05 AM
Old 03-11-2017
Hi,

Firstly, from just looking at a single IP, you can't tell how large the network it's a part of is. You need to know the netmask. So for example, if I had an IP of 192.168.1.10, that doesn't tell you anything at all about how large or small the network I'm a part of is.

But if I tell you the netmask - e.g. 192.168.1.10/24 or 192.168.1.10/255.255.255.0 - then you know it's part of a network of 256 addresses, running from 192.168.1.0 to 192.168.1.255.

If on the other hand I'd told you the netmask was 192.168.1.10/25 or 192.168.1.10/255.255.255.128 - then you'd know it was part of a network of 128 addresses going from 192.168.1.0 to 192.168.1.127.

And again, in every network, the very first address and the very last address are not usable for hosts, as they are reserved for other purposes.

You can also get plenty of other netmasks than the old simple Class A, B, C notation tells you. For instance I could just have easily told you my netmask was 192.168.1.10/28, in which case you'd then be able to calculate that the network I was a part of runs from 192.168.1.0 to 192.168.1.15, and conists of 16 IP addresses.

So just from looking at an IP you can't tell anything about the size of the network. And these days, there are far more netmasks and subnet sizes that are far more common than the old Class A/B/C notation.
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SHOREWALL-EXCLUSION(5)						  [FIXME: manual]					    SHOREWALL-EXCLUSION(5)

NAME
exclusion - Exclude a set of hosts from a definition in a shorewall configuration file. SYNOPSIS
!address-or-range[,address-or-range]... !zone-name[,zone-name]... DESCRIPTION
The first form of exclusion is used when you wish to exclude one or more addresses from a definition. An exclaimation point is followed by a comma-separated list of addresses. The addresses may be single host addresses (e.g., 192.168.1.4) or they may be network addresses in CIDR format (e.g., 192.168.1.0/24). If your kernel and iptables include iprange support, you may also specify ranges of ip addresses of the form lowaddress-highaddress No embedded whitespace is allowed. Exclusion can appear after a list of addresses and/or address ranges. In that case, the final list of address is formed by taking the first list and then removing the addresses defined in the exclusion. Beginning in Shorewall 4.4.13, the second form of exclusion is allowed after all and any in the SOURCE and DEST columns of /etc/shorewall/rules. It allows you to omit arbitrary zones from the list generated by those key words. Warning If you omit a sub-zone and there is an explicit or explicit CONTINUE policy, a connection to/from that zone can still be matched by the rule generated for a parent zone. For example: /etc/shorewall/zones: #ZONE TYPE z1 ip z2:z1 ip ... /etc/shorewall/policy: #SOURCE DEST POLICY z1 net CONTINUE z2 net REJECT /etc/shorewall/rules: #ACTION SOURCE DEST PROTO DEST # PORT(S) ACCEPT all!z2 net tcp 22 In this case, SSH connections from z2 to net will be accepted by the generated z1 to net ACCEPT rule. In most contexts, ipset names can be used as an address-or-range. Beginning with Shorewall 4.4.14, ipset lists enclosed in +[...] may also be included (see shorewall-ipsets[1] (5)). The semantics of these lists when used in an exclusion are as follows: o !+[set1,set2,...setN] produces a packet match if the packet does not match at least one of the sets. In other words, it is like NOT match set1 OR NOT match set2 ... OR NOT match setN. o +[!set1,!set2,...!setN] produces a packet match if the packet does not match any of the sets. In other words, it is like NOT match set1 AND NOT match set2 ... AND NOT match setN. EXAMPLES
Example 1 - All IPv4 addresses except 192.168.3.4 !192.168.3.4 Example 2 - All IPv4 addresses except the network 192.168.1.0/24 and the host 10.2.3.4 !192.168.1.0/24,10.1.3.4 Example 3 - All IPv4 addresses except the range 192.168.1.3-192.168.1.12 and the network 10.0.0.0/8 !192.168.1.3-192.168.1.12,10.0.0.0/8 Example 4 - The network 192.168.1.0/24 except hosts 192.168.1.3 and 192.168.1.9 192.168.1.0/24!192.168.1.3,192.168.1.9 Example 5 - All parent zones except loc any!loc FILES
/etc/shorewall/hosts /etc/shorewall/masq /etc/shorewall/rules /etc/shorewall/tcrules SEE ALSO
shorewall(8), shorewall-accounting(5), shorewall-actions(5), shorewall-blacklist(5), shorewall-hosts(5), shorewall_interfaces(5), shorewall-ipsets(5), shorewall-maclist(5), shorewall-masq(5), shorewall-nat(5), shorewall-netmap(5), shorewall-params(5), shorewall-policy(5), shorewall-providers(5), shorewall-proxyarp(5), shorewall-rtrules(5), shorewall-routestopped(5), shorewall-rules(5), shorewall.conf(5), shorewall-secmarks(5), shorewall-tcclasses(5), shorewall-tcdevices(5), shorewall-tcrules(5), shorewall-tos(5), shorewall-tunnels(5), shorewall-zones(5) NOTES
1. shorewall-ipsets http://www.shorewall.net/manpages/shorewall-ipsets.html [FIXME: source] 06/28/2012 SHOREWALL-EXCLUSION(5)
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