Get all IP addresses from a given IP address and subnet mask

To include NetworkAddress and BroadcastAddress

import org.apache.commons.net.util.*;

 SubnetUtils utils = new SubnetUtils("192.168.1.0/28");
       utils.setInclusiveHostCount(true);

       String[] allIps = utils.getInfo().getAllAddresses();

The IPAddress Java library supports both IPv4 and IPv6 subnets in a polymorphic manner. Disclaimer: I am the project manager.

Here is sample code to list the addresses for an IPv4 or Ipv6 subnet transparently. Subnets can get quite large, especially with IPv6, and it is not wise to attempt to iterate through a large subnet, so the code for iterateEdges shows how to iterate through just the beginning and ending addresses in the subnet.

show("192.168.10.0/24");
show("2001:db8:abcd:0012::/64");

static void show(String subnet) throws AddressStringException {
    IPAddressString addrString = new IPAddressString(subnet);
    IPAddress addr = addrString.toAddress();
    show(addr);
}

static void show(IPAddress subnet) {
    Integer prefixLength = subnet.getNetworkPrefixLength();
    if(prefixLength == null) {
        prefixLength = subnet.getBitCount();
    }
    IPAddress mask = subnet.getNetwork().getNetworkMask(prefixLength, false);
    BigInteger count = subnet.getCount();
    System.out.println("Subnet of size " + count + " with prefix length " + prefixLength + " and mask " + mask);
    System.out.println("Subnet ranges from " + subnet.getLower() + " to " + subnet.getUpper());
    int edgeCount = 3;
    if(count.compareTo(BigInteger.valueOf(256)) <= 0) {
        iterateAll(subnet, edgeCount);
    } else {
        iterateEdges(subnet, edgeCount);
    }
}

Iterates through entire subnet, use with caution:

static void iterateAll(IPAddress subnet, int edgeCount) {
    BigInteger count = subnet.getCount();
    BigInteger bigEdge = BigInteger.valueOf(edgeCount), currentCount = count;
    int i = 0;
    for(IPAddress addr: subnet.getIterable()) {
        currentCount = currentCount.subtract(BigInteger.ONE);
        if(i < edgeCount) {
            System.out.println(++i + ": " + addr);
        } else if(currentCount.compareTo(bigEdge) < 0) {
            System.out.println(count.subtract(currentCount) + ": " + addr);
        } else if(i == edgeCount) {
            System.out.println("...skipping...");
            i++;
        }
    }
}

Iterates through subnet edges:

static void iterateEdges(IPAddress subnet, int edgeCount) {
    for(int increment = 0; increment < edgeCount; increment++) {
        System.out.println((increment + 1) + ": " + subnet.getLower().increment(increment));
    }
    System.out.println("...skipping...");
    BigInteger count = subnet.getCount();
    for(int decrement = 1 - edgeCount; decrement <= 0; decrement++) {
        System.out.println(count.add(BigInteger.valueOf(decrement)) + ": " + subnet.getUpper().increment(decrement));
    }
}

Here is the output:

Subnet of size 256 with prefix length 24 and mask 255.255.255.0
Subnet ranges from 192.168.5.0/24 to 192.168.5.255/24
1: 192.168.5.0/24
2: 192.168.5.1/24
3: 192.168.5.2/24
...skipping...
254: 192.168.5.253/24
255: 192.168.5.254/24
256: 192.168.5.255/24

Subnet of size 18446744073709551616 with prefix length 64 and mask ffff:ffff:ffff:ffff::
Subnet ranges from 2001:db8:abcd:12::/64 to 2001:db8:abcd:12:ffff:ffff:ffff:ffff/64
1: 2001:db8:abcd:12::/64
2: 2001:db8:abcd:12::1/64
3: 2001:db8:abcd:12::2/64
...skipping...
18446744073709551614: 2001:db8:abcd:12:ffff:ffff:ffff:fffd/64
18446744073709551615: 2001:db8:abcd:12:ffff:ffff:ffff:fffe/64
18446744073709551616: 2001:db8:abcd:12:ffff:ffff:ffff:ffff/64

The following is the same as Sean's (really nice!) answer using https://seancfoley.github.io/IPAddress/ , it is only reducing the signal to noise ratio:

subnetToIps("192.168.10.0/28");

    public void subnetToIps(String ipOrCidr) {
        IPAddressString addrString = new IPAddressString(ipOrCidr, IPAddressString.DEFAULT_VALIDATION_OPTIONS);
        IPAddress subnet = addrString.toAddress();
        System.out.println("Subnet ranges from " + subnet.getLower() + " to " + subnet.getUpper());

        int i = 0;
        for (IPAddress addr : subnet.getIterable()) {
            System.out.println(++i + ": " + addr);
        }
    }