isとは何を示しているか

下記構文は、手入力でIPv4を入れて出力するのですが。
構文内にある『is』とはなにを示していますか？

is >> b1 >> d1 >> b2 >> d2 >> b3 >> d3 >> b4;
は、なにをしていますか？

また、
constexpr unsigned long to_ulong() const noexcept
は必要ですか？

#include <iostream>
#include <array>
#include <sstream>
#include <assert.h>

class ipv4
{
    std::array<unsigned char, 4> data;
public:
    constexpr ipv4() :data{ {0} } {}
    constexpr ipv4(unsigned char const a, unsigned char const b,
        unsigned char const c, unsigned char const d) :
        data{ {a,b,c,d} } {}
    explicit constexpr ipv4(unsigned long a) :
        data{ { static_cast<unsigned char>((a >> 24) & 0xFF),
                static_cast<unsigned char>((a >> 16) & 0xFF),
                static_cast<unsigned char>((a >> 8) & 0xFF),
                static_cast<unsigned char>(a & 0xFF) } } {}
    ipv4(ipv4 const& other) noexcept : data(other.data) {}
    ipv4& operator=(ipv4 const& other) noexcept
    {
        data = other.data;
        return *this;
    }

    std::string to_string() const
    {
        std::stringstream sstr;
        sstr << *this;
        return sstr.str();
    }

    constexpr unsigned long to_ulong() const noexcept
    {
        return
            (static_cast<unsigned long>(data[0]) << 24) |
            (static_cast<unsigned long>(data[1]) << 16) |
            (static_cast<unsigned long>(data[2]) << 8) |
            static_cast<unsigned long>(data[3]);
    }

    friend std::ostream& operator<<(std::ostream& os, const ipv4& a)
    {
        os  << static_cast<int>(a.data[0]) << '.'
            << static_cast<int>(a.data[1]) << '.'
            << static_cast<int>(a.data[2]) << '.'
            << static_cast<int>(a.data[3]);
        return os;
    }

    friend std::istream& operator>>(std::istream& is, ipv4& a)
    {
        char d1, d2, d3;
        int b1, b2, b3, b4;
        is >> b1 >> d1 >> b2 >> d2 >> b3 >> d3 >> b4;
        if (d1 == '.' && d2 == '.' && d3 == '.')
            a = ipv4(b1, b2, b3, b4);
        else
            is.setstate(std::ios_base::failbit);

        return is;
    }
};

int main()
{
    ipv4 a(168, 192, 0, 1);
    std::cout << a << std::endl;
    std::cout << a.to_string() << std::endl;

    ipv4 b = a;
    ipv4 c;
    c = b;

    ipv4 ip;
    std::cout << ip << std::endl;
    std::cin >> ip;
    if (!std::cin.fail())
        std::cout << ip << std::endl;
}